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NASA SP-329 


A Pictorial Presentation by 

the Mariner 9 Television Team 

and the Planetology Program 

Principal Investigators 


Scientific and Technical Information Office 



Washington, D.C. 

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Library of Congress Catalog Card No. 73-6001 'i! 

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U.S. Government Printing Office. Wa.shintiton, D.C. 20402 

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In the annals of space exploration, a very particular 
place must be reserved for a 546-kg metal object that, 
tumbling and silent now, is encircling an alien planet 
hundreds of millions of kilometers from its native Earth. 
It will remain, we think, in this remote orbit for at least 
half a century — unless perhaps some earthbound caravel 
of the far future picks it up for return to a place of 
honor in the land of its origin. This metal object is the 
Mariner 9 spacecraft, a singularly responsive mechanism 
at the end of an exiguous electromagnetic link that per- 
formed one of the most remarkable missions in the his- 
tory of planetary exploration. 

From the beginning — before, in fact — it was an ex- 
ceptional mission. A twin spacecraft, programmed to do 
half the mapping and scientific reconnaissance of the 
planet Mars, died at birth, a victim of launch-vehicle 
failure. The Mariner team, working coolly under difficult 
constraints, rebuilt the flight plan and orbit of Mariner 9 
to accomplish as many as possible of the scientific objec- 
tives of both missions. Then, after 157 days of inter- 
planetary flight, the spacecraft arrived at Mars and suc- 

cessfully entered orbit — becoming the first human artifact 
ever to orbit another planet — only to encounter a planet- 
wide dust storm that was veiling the surface of Mars. 
Although this forced postponement of the mapping mis- 
sion for many weeks, it did provide an excellent opportu- 
nity to study the storm beneath. After the storm abated, 
Mariner 9 set about a mapping and scientific reconnais- 
sance of exceptional quality and value. It photographed 
virtually the whole surface of the planet, sent more than 
7000 images back to Earth, and relayed a total of more 
than 30 billion bits of information, an amount equivalent 
to 36 times the entire text of the Encyclopaedia Britan- 
nica. This is incomparably more than had been received 
from all earlier planetarv missions together. The pictures 
in this volume, which is but one of many scientific re- 
ports to derive from the mission, provide in their view 
of canyons and giant crevasses, craters and volcanoes, a 
new and exciting understanding of the red planet. 

James C. Fletcher. Administrator 

National Aeronautics and Space Administration 


Mariner 9 was launched from Kennedy Space Center 
on May 30. 1971. A midcourse maneuver on June 5 
placed its aiming point so close to Mars that no addi- 
tional course correction was necessary. The spacecraft 
was successfully inserted into Mars orbit on November 14 
at 00:15:29 GMT, becoming the first manmade object to 
orbit another planet. 

Initiated in 1968, the Mariner Mars 1971 Program 
had called for two spacecraft to orbit Mars during the 
1971 opportunity, one in a high inclination orbit and the 
other in a low inclination orbit. After Mariner o was lost 
during launch on May 9. the operational strategy was 
changed to an intermediate inclination orbit to achieve 
maximum scientific return from a single orbiter. The 
objective of the mission was to explore Mars from orbit 
for a period of time sufficient to observe a large fraction 
of the surface and to examine selected areas for dynamic 
changes. Imagery of the surface was to be obtained as 
well as significant data on the atmosphere and surface 

Eleven Principal Investigators were concerned with 
the six experiments carried by Mariner 9: 

Television — H. Masursky (team leader). U.S. Geological 
Survey, Flagstaff; G. Briggs, Jet Propulsion Labora- 
tory; G. De Vaucouleurs. University of Texas; J. Led- 
erberg. Stanford University: B. Smith. New Mexico 
State University. 

Ultraviolet spectroscopy — C. Barth, University of Colo- 

Infrared spectroscopy — R. Hanel, NASA Goddard Space 
Flight Center. 

Infrared radiometry — G. Neugebauer. California Institute 
of Technology. 

S-band occultation — A. Kliore, Jet Propulsion Laboratory. 

Celestial mechanics — J. Lorell (team leader), Jet Propul- 
sion Laboratory; I. Shapiro. Massachusetts Institute of 

The spacecraft was approaching Mars, when tele- 
scopes on Earth revealed that a planetwide dust storm had 
broken out and was totallv obscuring its surface. From 
November to mid-December only faint markings appeared 
on the surface of Mars and sometimes a diffuse feature 
with a series of billowing dust waves on its lee side. The 
last picture taken before orbital insertion had shown four 
curious dark spots aligned in a T-shaped pattern, and it 
was theorized that thev might be high-standing parts of 
an otherwise obscured planet. This area was monitored 
repeatedly during the course of the storm, and as succes- 
sive pictures showed more and more detail it became 
clear to the science team that these were the summit areas 
of enormous volcanoes protruding through the top of the 
dust cloud. By the end of December it appeared that the 
dust storm was diminishing and that the planetary map- 
ping sequences could soon begin. 

From January 1972 onward, every week was punc- 
tuated by new and startling discoveries. First there were 
the enormous volcanoes standing as much as 15 miles 
above the average surface, each one about the size of 

Arizona. Then, totally unanticipated, immense canyons 
appeared, including a great equatorial chasm more than 
ten times the size of the U.S. Grand Canyon. The canyons 
proved to have eroded walls, and in addition numerous 
dendritic tributaries extended back from the canyon walls, 
suggesting that water erosion may have played a role in 
sculpturing the surface of Mars some time in its past. Yet 
it was known from previous flyby missions that atmos- 
pheric and surface temperature conditions are such as to 
prevent liquid water from existing in adequate quantity at 
the present time. For this reason the science team was 
astounded by the apparent evidences of erosion, and then 
by the discovery of non-canyon-related sinuous channels 
that had all the earmarks of dry river valleys. Eroded 
cliffs appeared, as well as wind-erosion features and large 
dune masses. It is difficult to convey the sense of high 
excitement that pervaded the scientific investigators as 
the newly perceived character of our sister planet began 
to unfold. 

Soon it became apparent that almost all generaliza- 
tions about Mars derived from Mariners 4, 6, and 7 
would have to be modified or abandoned. The partici- 
pants in earlier flyby missions had been victims of an 
unfortunate happenstance of timing. Each earlier space- 
craft ( except in part for Mariner 7. which had returned 
startling pictures of the south polar regions) had chanced 
to fly by the most lunar-like parts of the surface, return- 
ing pictures of what we now believe to be primitive, 

cratered areas. Given a difference of as little as six hours 
in arrival times of any of these earlier spacecraft (each of 
which had spent many months in transit), an entirely 
different view of Mars would have resulted. It was almost 
as if spacecraft from some other civilization had flown by 
Earth and chanced to return pictures only of its oceans. 

Mars moved behind the Sun in early August 1972, 
and the spacecraft could no longer be commanded from 
Earth. At this point in the mission nearly all the planet 
had been mapped with the low resolution camera, and 
about 2 percent of its surface covered by the high resolu- 
tion camera, specially targeted over points of high scien- 
tific interest. In addition, the waning of the south polar 
cap had been examined in detail, and the layered and 
pitted deposits in these regions extensively pictured. At an 
altitude of 1650 km the resolution of the TV camera sys- 
tem was about 1 km for the low resolution camera and 
about 100 m for the high resolution camera. 

When Mars came out from behind the solar corona 
on October 12, so that scientific operations with the 
orbiter could be resumed, mapping coverage of the north- 
ern latitudes was completed and the northern polar re- 
gions examined in detail. After a lifetime in space of 
516 days, the Mariner 9 spacecraft ran out of attitude- 
control gas and tumbled out of control on October 27, 
1972, almost one year after it had been inserted into Mars 
orbit. — J. F. McCauley, H. F. Hipsher. and R. H. Stein- 


J. W. Allingham 

U.S. Geological Survey 


G. A. Briggs 

Jet Propulsion Laboratory 

M. H. Carr 

U.S. Geological Survey 
Menlo Park 

S. E. Dwornik 
NASA Headquarters 

W. E. Elston 

University of New Mexico 

J. C. Fletcher 
NASA Headquarters 

P. L. Fox 

Cornell University 

D. E. Gault 

NASA Ames Research Center 

M. Gipson, Jr. 
Virginia State College 

R. Greeley 

NASA Ames Research Center 

M. J. Grolier 

U.S. Geological Survey 

N. W. Hinners 
NASA Headquarters 

H. F. Hipsher 
NASA Headquarters 

H. E. Holt 

U.S. Geological Survey 


J. H. Howard HI 
University of Georgia 

K. A. Howard 

U.S. Geological Survey 
Menlo Park 

E. A. King, Jr. 
University of Houston 

J. S. King 

State University of New York 


T. J. Kreidler 

U.S. Geological Survey 

C. B. Leovy 
University of Washington 

J. F. McCauIey 

U.S. Geological Survey 

D. T. McClelland 
Hamilton College 

T. R. McGetchin 

Massachusetts Institute of Technology 

G. E. McGill 

University of Massachusetts 

J. D. Murphy 

State University of New York 


H. Masursky 

U.S. Geological Survey 


E. C. Morris 

U.S. Geological Survey 

T. A. Mutch 
Brown University 

J. E. Peterson 
University of Colorado 

J. B. Pollack 

NASA Ames Research Center 

D. B. Potter 
Hamilton College 

L. Quam 

Stanford University 

C. Sagan 
Cornell University 

R. S. Saunders 

Jet Propulsion Laboratory 

D. H. Scott 

U.S. Geological Survey 

R. P. Sharp 

California Institute of Technology 

E. M. Shoemaker 

California Institute of Technology 

B. A. Smith 

New Mexico State University 

L. A. Soderblom 

U.S. Geological Survey 


R. H. Steinbacher 

Jet Propulsion Laboratory 

J. Veverka 
Cornell University 

D. E. Wilhelms 

U.S. Geological Survey 

Menlo Park 

J. F. Woodruff 
University of Georgia 








Giant Volcanoes of Mars 



Mysterious Canyons 






Fractures and Faults 






Fretted and Chaotic Terrains 






Wind-Shaped Features 



Changing Features 



Extensive Plains 



Polar Regions 



Clouds of Mars 



Natural Satellites 



Martian Enigmas 



Similarities; Mars, Earth, and Moon 


Availability of Photographic Prints 


Shaded Relief Map of Mars 



Although the dust storm delayed the start of system- 
atic mapping, it afforded an unparalleled opportunity to 
examine its effects on the surface and atmosphere of Mars. 
Pictures of the limb were taken showing that dust reached 
the enormous elevation of about 70 km (43 mi.). Grad- 
ually features emerged through the haze. At first only the 
dimly shining south polar cap and four dark spots could 
be seen. One of the dark spots had been noted during the 
dust storms of 1924' and 1956 by astronomers. Lnder nor- 
mal conditions this feature appears as a bright white spot, 
Olympus Mons. The other three spots lay in the area 
where periodic brightenings called the "W-cloud" have 
often appeared. As the storm gradually subsided and the 
atmosphere cleared, the four spots turned out to be high 
mountains with craters at their summits. Olympus Mons 
appeared as an immense shield volcano 24 km high with 
long finger-shaped lava flows on its flanks — the largest 
volcanic pile ever photographed. Later a great plateau 
became visible, sloping to the east from the volcanoes. 
On it appeared a bright stripe that later turned out to be 
a great equatorial chasm. 

The more than 7300 pictures acquired from Mariner 
9 indicate that Mars is more varied and dynamic than 
previously inferred. Although impact craters are common, 
only a few small craters show continuous ejecta blankets 
and well developed rays. Most small craters, however, 
exhibit degraded, irregular ejecta blankets. About half 
the surface consists of ancient cratered terrain surround- 
ing large impact basins. The largest circular feature. Hel- 
las Planitia, is almost twice the size of the largest basin 

on the Moon. Mare Imbrium. Argyre Planitia is ringed 
by radially and concentrically textured mountainous ter- 
rain, similar to the lunar multi-ringed impact basins such 
as Imbrium and Orientale. The remainder of the surface 
is covered by younger volcanic rocks and volcanoes. These 
rise as much as 25 km above the mean level of extensive 
lava plains deposits, some of which contain windblown 
or possibly fluviatile deposits that are sedimentary in 
origin. The single volcanic edifice of Olympus Mons, 
which rises high above the floor of Amazonis Planitia. is 
almost three times the width and height of the largest of 
the Hawaiian volcanoes. Mauna Loa. Three other large 
volcanoes lie along the Tharsis ridge. The volcanoes with 
summit calderas have fresh flows on their slopes and ap- 
pear to be relatively young. These volcanic vents provide 
a plausible source for much of the carbon dioxide and 
water in the atmosphere. The great equatorial chasm or 
canyon svstem. Valles Marineris, comparable in size to 
the East African Rift Vallev svstem, is as much as 6 km 
deep and greater than 5000 km long, the distance from 
Los Angeles to New York City. It terminates in a com- 
plexly faulted plateau to the w'est, and in large patches of 
chaotic terrain to the east. 

Emerging from the northern plateau lands, a com- 
plex array of broad sinuous channels descends into a 
regionally depressed area. Large fluvial channels begin in 
this chaotic terrain — possibly from episodic melting of 
permafrost — and seem to flow northw ard into the Chryse 
Planitia lowland. The channels merge on the border of 
the flat, low Chryse area: here the channel floors show 

multiple braided features and streamlined islands. It has 
been proposed that the collapse of these rocks and forma- 
tion of large-scale landslides may be caused by melting 
of permafrost. 

Other large sinuous channels with many tributaries 
have no obvious sources. Small dendritic channel net- 
works abound in the equatorial regions and imply pos- 
sible rainfall. Many of the basin floors are underlain by 
lava flows having lobate fronts, and are inferred to be 
basaltic from the form of the flows, ridges, and broad, 
low mare-type domes that characterize their surface. 

The polar regions are covered by glacio-eolian lay- 
ered rocks that appear to be still forming under the pres- 
ent climatic regime. Older massive deposits are being 
eroded, pitted, and etched into troughs around the mar- 
gins of the poles. Young layered deposits resembling thin 
laminae overlie the etch-pitted unit. The individual thin 
layers appear to be cyclical deposits. High velocity wind 
is stripping the surface and forming deflation hollows. A 
mantle of ^vindblo\vn debris, presumably derived from 
these circumpolar zones, thins toward the equator. These 
deposits smoothly blanket a subdued cratered terrain and 
partially fill its craters. The south and north polar regions 
have a])parently acted as sediment or dust traps through- 
out much of Mars history. 

Both eolian erosional features such as yardangs 
(wind eroded ridges I and depositional features such as 
dunes have been identified in the equatorial region. One 
dune field, about 130 km long, lies on the floor of a 
crater. Wind erosional and depositional processes are ac- 

tive, as seen by numerous changes in the albedo patterns 
that were monitored after the clearing of the planetwide 
dust storm. Redistribution of deposits of silt and clay 
particles reveals dark, irregular markings and light and 
dark tails emanating from topographic obstacles. The 
light tails appear to be wind-deposited material: the dark 
tails appear to be mostly wind-scoured zones. Throughout 
the mission clouds of various patterns composed of CO2 
ice crystals, water ice crystals, and local wind raised dust 
clouds were observed. 

The temperature measurements and cloud patterns 
led to interpretations of the planetwide atmospheric cir- 
culation pattern, which in turn could be compared with 
the bright and dark surface markings that also indicate 
wind directions. Changes in the surface patterns were 
monitored on a periodic basis. During this time the dark 
markings that had been, observed from Earth telescopes 
for more than a hundred years gradually reappeared 
after having been obscured by the storm deposits. 

The retreats of both the north and south polar ice 
caps were observed closely. The carbon dioxide and pos- 
sibly some water ice retreated by sublimation, revealing 
layered deposits formed by glacial-like processes, and a 
belt of etched pitted terrain surrounding the polar ice-cap 
region. The hollows may be formed by wind erosion, for 
the winds at the margins of the polar caps have a very 
high velocity on Mars, as they do on Earth in Antarctica 
and near the Greenland ice cap. 

The spacecraft ceased functioning when it ran out of 
attitude-control gas after .349 days in orbital operation. 

It succeeded its design lifetime by almost a factor of four. Mars in 1975-76 that involve landing spacecraft on the 

and its observations exceeded all science goals. Mariner 9 surface of Mars to search for life. — H. Masursky and 

data will greatly assist planning for the Viking flights to B. A. Smith 


Giant Volcanoes of Mars 

Recognition of prominent volcanic features on Mars 
was one of the first and most significant results of the 
flight of Mariner 9. During the fully developed dust storm. 
the only surface features clearly visible outside the polar 
areas were four dark spots in the Amazonis-Tharsis re- 
gion. As the atmosphere cleared, those spots were seen 
to be the central calderas of four enormous shield vol- 
canoes. Subsequent photography of other parts of the 
planet revealed more volcanic features, indicating that 
volcanism played a major role in the evolution of Mars. 
Past volcanic activity includes formation of extensive 
plains units, and building of the tremendous shield vol- 
canoes and numerous smaller dome-like structures. 

Most of the volcanic features except the plains are 
in the regions of high elevation. The three shield volca- 
noes, the Tharsis Monies, lie on a broad ridge which is 
3 to 5 km above the mean level of the martian surface. 
Olympus Mons. the largest of the volcanic shields, lies 
on the western flank of this ridge. Olympus is 500 km 
wide and rises 29 km above the surrounding plain. The 
Tharsis Monies, Ascraeus. Pavonis, and Arsia Mons are 
each about 400 km across and. although smaller than 
Olympus Mons, may reach the same elevation above the 
mean level of Mars because of their location on a ridge. 
In comparison, the largest volcano on Earth. Mauna Loa 
in Hawaii, is approximately 200 km wide and rises about 
9 km above the sea floor. 

All shield volcanoes have roughly circular outlines 

and central summit depressions. Arsia Mons, Pavonis 
Mons. and an Elysium shield. Albor Tholus, have simple 
craters at their summits. Olympus Mons and Ascraeus 
Mons have complex craters as a result of successive col- 
lapses around different centers. Other volcanoes, differing 
from shield volcanoes in that they are smaller and simple, 
are properly termed domes or tholi. 

The shields and domes are the most spectacular 
aspects of martian volcanism, but the plains on Mars 
may be volumetrically more significant. High resolution 
pictures of the plains commonly show long, low, lobate 
scarps (possible flow fronts) that strongly resemble fea- 
tures in Mare Imbrium on the Moon. By analogy with the 
lunar maria and terrestrial flow fronts, the plains are 
probably largely volcanic in origin. 

In many places the cratered surface appears to be 
partly or wholly covered by younger plains-forming mate- 
rials. In some areas only the small craters are buried, in 
others even the largest craters are buried entirely or show 
only subdued impressions. Such effects could result from 
eolian deposition, but volcanic activity also appears to 
have been widespread and products of this activity also 
may cover part of the cratered surface. Both volcanic 
plains and circular constructional features are found 
within the densely cratered province. Thus, although the 
most spectacular volcanic features occur in sparsely 
cratered regions, the entire planet may have been affected 
by volcanism. — M. H. Carr 


(20°N, 135°W: MTVS 4133-96) 

Long lava flows (above left) are visible in this photograph of the northwest flank of 
Olympus Mons (resolution, about 100 m). Many show natural levees such as occur 
along the margins of many terrestrial lava flows. The most prominent ridge has a 
channel ( arrow I 250 m wide along 36 km of its crest that is inferred to be a lava chan- 
nel. Lava flows of this form are characteristic of basaltic eruptions in the Hawaiian 
and Galapagos Islands on Earth. — H. Masursky 

(18°N, 133°W; MTVS 4265-52) 

The central caldera (above right) on Olympus Mons shows a structure of intersecting 
collapse depressions and concentric fractures. The inward collapse of the caldera 
floor is evident from the terrace pattern that steps toward the caldera center, a pattern 
similar to terrestrial volcanic calderas. The smaller, youngest, collapse pit (top center) , 
is about 30 km across. — H. Masursky 

(18°N, 133°W) 

Photomosaic of Olympus Mons (facing page), the largest of the Mars volcanic moun- 
tains. The volcanic structure is 500 km across and about 29 km high, with a complex 
summit caldera about 70 km across. These dimensions make it the largest volcanic 
structure known. It is much larger than the island of Hawaii, which (on the ocean 
floor ) at 200 km across and 9 km high is the largest volcanic pile on the Earth. The 
scarp around the base of Olympus Mons stands 1 to 4 km high and may have been 
produced by wind erosion. Originally the volcanic pile probably graded smoothly into 
the surrounding plain. — H. Masursky 








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(1°N, 113°W; MTVS 4267-44) , 

The central crater and ring structure of Pavonis Mons are shown in this oblique view 
(above). The smooth crater-free floor and talus on the walls of the summit pit, and a 
series of collapse terraces at the sides, are clearly visible. Radial ridges, similar to lunar 
mare ridges, connect the central pit to the ring structure of grabens and horst ridges. 
The dark patches formed during the mission and were almost certainly produced by 
eolian processes. — M. H. Carr 

(1°N, 112°W; IPL 1699/125324) 

The shield volcano at Pavonis Mons (left) is about 400 km across and rises more than 
20 km above the surrounding plains. Concentric graben occur on the flanks of the 
shield and in the surrounding plains. The caldera consists of a single large circular 
depression. 55 km in diameter. — :M. H. Carr 


(1°N. 113°W; MTVS 4142-93) 

Part of the summit caldera of Pavonis Mons is shown here. The caldera-wall fluting is 
probably caused by debris avalanches cutting large grooves down the steep slope. 
Talus debris may overlie narrow terrace benches. The smooth caldera floor, which 
abruptly meets the steep walls, may represent the surface of a former lava lake. Well- 
defined impact craters with sharp rims ranging from 1/2 to 2 km are visible on the 
flanks of the volcano. — M. H. Carr 








(1°N, 113°W; IPL 7388/011543) 

The ridges around Pavonis Mons are here shown enlarged, revealing their similarity to 
lunar maria ridges. They are inferred to be extrusions of lava along a complex fracture 
system extending more than 30 km down the flanks of the shield volcano. The dark 
patches shown in a previous picture have not yet developed. — M. H. Carr 






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(13°N, 89°W; MTVS 4189-72) 

Tharis Tholus (above). 170 km in diameter, is one of several similar volcanic domes 
near the Tharsis Montes. The central crater is multiple, has a flat floor and steep walls 
with several terraces. The flanks of the dome appear to have been faulted ( upper right) . 
Domes may form instead of the larger and more gentle shield structures when only 
small volumes of lava are available. Alternativelv. they may indicate more viscous and 
possibly more siliceous lava. — M. H. Carr 

(13°N, 106° W; MTVS 4184^84) 

Ascraeus Mons (left), the northernmost large volcano along the crest of Tharis ridge, 
shows a complex summit caldera about 60 km across, the multiple overlapping craters 
and prominent terraces indicate the volcanic nature of the large mountains. Ascraeus 
Mons protruded through the planetwide dust storm as a dark spot, and in December 
1972 it became the first clearly identified volcanic structure on Mars. The revelation 
of volcanoes on Mars thus overturned the Mariner 4, 6. and 7 thesis that Mars was a 
dead planet. — H. Masursky 



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(9°S, 120°W: IPL 1633/004651. 492/141002) 

Arsia Mons (preceding page ) : a shield volcano in the Tharsis Monies. A central 
smooth-floored caldera 130 km in diameter is surrounded by a zone of concentric 
graben. Outside the faulted zone are numerous superimposed lava-flow lobes and 
sinuous channels with isolated graben areas. The flanks are partly embayed by the 
surrounding plains materials. The structure is believed to be similar to Olympus Mons 
but somewhat older. The flows are shorter and thicker than those on Olympus Mons, 
perhaps because of chemical differences, a lower gas content, or eruption at lower 
temperatures. These flows are more similar to those on the flanks of Mount Rainier and 
Mount Hood in the Pacific Northwest of the United States that are andesitic in compo- 
sition. — M. H. Carr and H. Masursky 


(10°S, 124°W: MTVS 4182-42) 

The southwest flank of the large volcanic shield Arsia Mens shows a rough, slightly 
cratered terrain with large lobate lava flows trending downslope. Wind erosion has 
etched the older flow fronts into a rougher terrain. The picture is about 32 km across. — 
T. R. McGetchin 

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(63°S, 323°W: MTVS 4238-7) 

The crater above, 20 km in diameter, may be of impact origin with subsequent modifi- 
cation by volcanism. The flat-bottomed depression in its middle appears to have 
formed by collapse. Its central peak or dome may be a volcanic cone, as may many 
of the other cone-like features nearby. Surrounding the crater are many small volcanic 
cones, ranging from 2 km down to the limit of resolution, here around 250 m. — 
J. E. Peterson 

(38°N. 196°W; MTVS 4244^75) 

A series of small domes or volcanic cones (left) rising from a flat plains terrain. The 
arcuate distribution of cones suggests extrusion along the fracture system of an old 
crater. Note the small crater on the summit of a cone (arrow). The cones are 3 to 7 
km in diameter at their bases. The intracone plain appears to consist of overlapping 
lava flows covered with a mantle of finer material (windblown debris or volcanic ash) 
which subdues the flow fronts and other relief features. — D. B. Potter 








(22°N, 97°W: MTVS 4187-90 ) 

Three volcanic domes (left) near Alba Patera. The dome of Uranius Patera (upper 
left) has collapsed, creating a large complex caldera. Ceraunius Tholus has a sinuous 
channel leading down from the caldera to a closed depression at the base of the dome. 
A third dome. Uranius Tholus, is seen at left center. Note the series of parallel, closely 
spaced fault valleys in the bottom of the photo. — D. B. Potter 

(24°N, 98° W: MTVS 4271-.51) 

A sinuous channel, about 1 km wide, occurs on the flank of the volcanic cone Ceraunis 
Tholus. The summit caldera wall was breached and the channel eroded when fluids 
drained from the caldera basin (off right) to the closed depression at the foot of the 
cone. The mouth of the 40-km sinuous channel seems to grade into a deltalike deposit. 
Many smaller sinuous channels cross the flanks of the dome, and several channels show 
distributary deposits at their lower ends. Presumably, the channels are related to vol- 
canic activity, but their overall characteristics are also similar to fluvial channels. — 
H. E. Holt 

(25°N, 213° W; MTVS 4298-44) 

Elysium Mons is a symmetrical shield volcano (above) approximately 225 km across, 
with a small central caldera and numerous fractures radial and concentric to the shield. 
Several channels and lines of craters in the flanks of the shield appear in high resolu- 
tion photographs. Two incomplete concentric fracture rings surround the shield, one 
at a radius of 175 km and one at 320 km. Similar concentric fracture systems occur 
around other Mars shield volcanoes. — M. H. Carr 

(25°N, 213°W; IPL 7386/014900, 7386/020050) 

The summit area of the Elysium volcanic cone (right) shows a well defined radial 
pattern of material on the slopes surrounding the central crater. Several small chains 
of rimless pits are on the right flank of the cone. The crater rim is broken by several 
sinuous lava channels. The features in line with the lava channels in the lower part of 
the photo are possibly collapsed lava tubes. The flat floor of the crater suggests that 
it contained a lava lake. — J. W. Allingham 


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{31°N, 210°W: MTVS 4298^7) 

A small caldera (left) 10 km wide on the flanks of Elysium Mons. The caldera shows 
multiple collapse depressions and several fine channels. Lines of small volcanic craters 
are arranged along radiating fractures (lower right). Like the sinuous rilles on the 
Moon, these lava channels start in a source crater and become narrower and shallower 
downslope. Terrestrial iaval channels have similar forms. — H. Masursky 

(32°N. 211°W: IPL 7386/023416) 

The edge of the Elysium dome shows relationships that typify the contacts of volcanic 
domes with the surrounding plains. A low escarpment may occur as in the bottom of 
the frame, or the radial channels on the flanks may be truncated when they dip be- 
neath the surrounding materials as in the upper center of the picture. Low escarp- 
ments outline a series of lobate flow sheets extending from a crater (probably volcanic) 
about 9 km across. The lobate flows are very similar to basaltic lava flows on the Earth 
and Moon. — M. H. Carr 



Mysterious Canyons 

One of the most spectacular revelations of Mariner 9 
was the system of huge canyons in the equatorial region 
of Mars. These extraordinary features, up to 200 km 
wide, thousands of kilometers long, and possibly as much 
as 6 km deep, represent a significant phase in the planet "s 

The system of canyons, Valles Marineris, extends 
5000 km along the equatorial belt. Some of the dark 
markings that have been mapped for a century from 
terrestrial telescopes coincide with the floors and walls 
of these huge canyons. The nature of these markings re- 
mained hidden until thev were pictured by Mariner 9. 

The canyons consist of a series of parallel depres- 
sions characterized by steep gullied walls and a sharp 
brink at the lip of each canyon. The elongation of indi- 
vidual depressions is parallel to the trend of the entire 
belt. Walls of the canyons are rarely smooth. Most of 
them exhibit features ranging from broad open embay- 
ments to complex branching ravines and gullies. Some of 
these gullies have dendritic drainage patterns and extend 
back into the surrounding uplands for distances of up to 
1.50 km. Knobs, spurs, and other irregularities suggest, 
along with different degrees of dissection, some degree of 
inhomogeneity in the material forming the canyon walls 
themselves. The canyon floors generally lack craters, sug- 
gesting either relative youth of the floors, or the effects 
of some erosional process that obliterates all traces of 

A moderate sprinkling of craters appears on the up- 
lands surrounding the canyons: some of these craters 
have broken, jumbled, and apparently downdropped 
floors. Another canyon-related feature is the presence of 
linear chains of rimless pits, probably of collapse origin. 
It seems that craters and pits predating the canyons have 
served at least partly as sites for downward collapse that 
lead to the formation of the small parallel canyons. 

What created the canyons? The parallelism of indi- 
vidual canyons and the parallel trends of pit chains and 
smaller fault valleys or graben implies a strong degree of 
control by regional structural patterns. The blunt ends of 
the canyons suggest that the widening and lengthening 
of them by wall recession must have been a factor in their 
formation. Jumbled masses of rocky debris piled on 
canyon floors at the bases of numerous U-shaped gullies 
indicate that mass slides, slumps, and debris avalanches 
must have been a factor in shaping the canyon walls. 

The major obstacle to any convincing explanation of 
the origin of the canyons is: How was the bulk of the 
material originally present in these enormous chasms 
removed? There is no obvious way to transport debris 
out except by wind. Yet the amount of material to be 
transported is so great as to cast doubt on the effective- 
ness of this mechanism operating by itself. The disposal 
of such vast amounts of material remains a problem. — 
J. F. McCaulev 










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The great canyon system, Valles Marineris, more than 5000 km long and at least 6 km 
deep, dwarfs any stream valleys on the continents of Earth. A minor side canyon is 
similar in length and depth to the Grand Canyon of Arizona (inset) . Features on Earth 
most closely comparable in size to Valles Marineris are the Rift Valley system of Africa 
and related rift valleys on ocean floors. As with Earth's rift valleys, Valles Marineris 
may have been formed where the crust of the planet has pulled apart. Pieces of the 
crust that form the floor of the canyon probably have subsided along faults. Subse- 
quently the rim of the valley has been sculptured by mysterious processes of erosion. 
The intricate system of canyons extending back from the rim may have been developed 
during melting and evaporation of subsurface ice. — E. M. Shoemaker 

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(8°S, 84°W; MTVS 4144-87) 

The s])ine-like ridge seen running through the center of the canyon above is located in the 
far canyon on the facing page. Also notable are angulate dendritic tributaries on the wall and 
large landslide alcoves (bottom). — R. P. Sharp 

(5°S,86°W; MTVS 4191-45) 

The parallel nature (right) of elements in the canyon system is revealed by this view of 
two canyons with scarred and gullied walls. A chain of pits on the remnant of upland sepa- 
rating the two also parallels the canyons. Landslide debris is evident in the canyon floor at 
bottom right. The surface reflectivity variation is due to changes in the slope near the rims 
of the canyons. This section of canyon is 440 km long. — D. T. McClelland 


(13°S, 61°W; IPL 1616/212555) 

This high resolution view, about 35 km in width, of the wall of Coprates Chasma 
shows ravines and narrow branching divides that lie beneath a series of sharp crested 
alcoves. Although these features seem to resemble at first glance typical "badlands," 
topography of the kind produced by episodic cloudbursts in arid regions, closer in- 
spection reveals another possible origin. The bottoms of the gullies are not intercon- 
nected and individual divides interrupt one another. Thus the pattern is not the same 
as that generally produced by running water but is more similar to that produced by 
mass wasting or gravity sliding of loose materials on oversteepened slopes. — J. F. 

(7°S, 85°W; IPL 1354/184219) 

The steep headwall. scarred by possible dry avalanche chutes at its rim. rises several 
kilometers above the jumbled landslide topography on the floor of the trough. The 
smooth band below the headwall may be accumulated talus deposits. Slides like this 
are locally common on trough walls. They might have resulted from undermining by 
removal of ground ice by evaporation or by melting under different climatic condi- 
tions. This image is about 42 km wide. — R. P. Sharp 

(5°S. 77°W: IPL 1628/204400) 

A blunt-ended trough (left) in Valles Marineris. Ophir Chasma, was captured in this 
magnificent picture, the width of which covers about 400 km. Swirl pattern on the 
floor of the trough may reflect outcrop of dissected floor deposits. Smaller troughs and 
lines of pits extending westward from the headwall suggest initiation of troughs along 
fractures or structures in crust. — R. P. Sharp 

(13°S. 110°W; IPL 1348/223600) 

The pits (above) at the right of this canyon suggest one possible method of enlarge- 
ment of the canyons by collapse and drainage of surface material into what must be 
a cavernous or porous subsurface. Thus the troughs may expand along lines of these 
pits as well as by erosion of the walls as seen in the vertical chutes here and in the 
other numerous examples of wall erosion seen in this section. This picture is about 
40 km in width. — J. W. Aliingham 

(22°S, 254°W; MTVS 4295-79) 

These box canyons (right), in Hesperia Planum, display parallel trends that suggest 
they may have developed along fractures. They were clearly formed before the large 
number of local small cratering events. — D. B. Potter 


(6°S, 105° W; MTVS 4187-45) 

This "labyrinth" occurs at the western end or origin of Valles Marineris as seen in this low 
resolution frame some 400 km across. It is characterized by smooth-walled gaping depres- 
sions and chain craters that partly surround large flat-topped mesas. Long, narrow linear 
graben also lace the area; many of these are cut bv the steep depressions. The grossly polyg- 
onal pattern of the chain craters and elongate depressions is very reminiscent of that pro- 
duced bv doming on Earth but it is very much larger in scale. This region is nearly coincident 
with a broad swelling of the Mars surface that appears to be several kilometers higher than 
the surrounding plains. — J. F. McCauley 

(1°S, 76°W; IPL 1628/210149) 

Deadend: This 300-km-long canyon (left) is completely enclosed. It lies somewhat to the 
north of Coprates Chasma. Ravines and gullies mark the wall on the right while the left wall 
has shallow alcoves with hummocky landslide material at the base. The uplands show a range 
of crater size and a set of parallel fractures. — J. W. Allingham 




(24°N, 62°W; IPL 1356/120125) 

A mesa-like plateau occurs in the Lunae Planum region. Prominent scarps separate it 
from adjoining lowlands, which are shown in regional pictures as an extensive valley 
complex. The regular scalloping along the upper edge of the scarps suggests headward 
mass wasting and eolian fluting. The plateau section shown here is about 60 km in 
length.— T. A. Mutch 




Numerous channels, ranging from broad sinuous 
channels nearly 60 km wide to small ( less than 100 m 
wide I narro'.v dendritic channel networks, occur over 
local and widespread martian regions. Many of the chan- 
nels appear remarkably similar to stream channels on 
Earth. Sinuous channels containing discontinuous mar- 
ginal terraces, teardrop-shaped islands, and braided 
stream channels and bars, must have been eroded by 

The channels of Mars have been grouped into four 
general types. Three types have characteristics that imply 
a fluvial origin: broad and sinuous channels, narrow 
channels with tributaries and braided streambeds, and 
closely spaced coalescent channels. A fourth variety has 
characteristics that imply molten lava channels. 

Some of the largest channels, which are 30 to 60 km 
wide and up to 1200 km long, appear to originate in the 
northern plateau lands and flow northward into the Chryse 
region. As the complex array of the broad, sinuous chan- 
nels empties into the flat low Chryse area, the channel 
floors show characteristics that confirm the northward 
direction of flow consistent with the regional slope of the 
surface. These channels resemble features produced by 
episodic floods on Earth. The large Chryse channels have 
potential sources of fluids in the chaotic terrain, and the 
tributaries are proportional in size to the area of chaotic 
terrain they drain. Catastrophic melting of ground ice 
could form both the chaotic terrain and the giant flood 
channels in a single event. 

Narrow, sinuous valleys, some with many tributaries 
forming dendritic-like patterns, lie on high level plateau 

surfaces such as Lunae Planum and Memnonia in the 
martian equatorial region. The fluvial character of these 
channels, combined with the lack of apparent source 
areas, requires the surface collection of fluids into inte- 
grated channels along with surface erosion and subse- 
quent deposition in alluvial basins. An intermittent at- 
mospheric source for channel erosion appears logical and 
is supported by the presence of channels which head very 
close to ridge crests. 

Local networks of very small coalescent channels are 
widely spaced across the equatorial region. Northwest of 
Hellas Planitia, networks of coalescent channels run down 
the sides of many craters. Their form again suggests a 
precipitation collection system and such an origin re- 
quires widespread intermittent precipitation across the 
equatorial zone. 

Another type of channel, associated with volcanic 
centers, is the lava channel or collapsed lava tube. These 
channels start on the flanks of volcanic domes and shield 
volcanoes but become less defined downslope. This rela- 
tionship is the opposite of that generally observed in 
stream channels. 

Most martian channels are indicative of past erosion, 
transport, and deposition of surface materials that only 
running water could produce. Under present martian at- 
mospheric conditions, liquids would not exist on the sur- 
face except during rare conditions. — H. E. Holt and 
M. A. Sheldon 




(6°S, 150° W; MTVS 4258-35, 4258-39) 

The photomosaic (above) of the lower part of the Amazonis channel in Mangala 
Vallis shows complex braiding such as streams produce in arid environments on Earth 
bv depositing suspended sediment rapidly and intermittently. The streamlining of the 
"islands" very strongly implies formation by running water. Patterns like this have 
not been observed in lava channels on the Earth or Moon. The cuspness of the channel 
floor indicates that it was formed in geologically recent times; other martian channels 
are cratered and degraded as though much older. The crater seen along the right 
margin is about 20 km in diameter. — H. Masursky 

(31°N, 229°W; IPL 1441/152627) 

The channel at left (about 45 km wide I represents a sinuous muUi-channel course 
containing discontinuous marginal terraces, teardrop-shaped islands (blunt ends face 
upstream) and macro braided channels. The character of this channel indicates that 
it might have been eroded by fluids. This channel arises in a hummocky area and per- 
haps the fluids resulted from melting of ground ice or permafrost. The only terrestrial 
examples of such large sinuous channels occur in the channeled scablands of the 
Columbia Plateau in the United States and the Sandier plains of Iceland, where release 
of great volumes of water resulted in catastrophic erosion. — -H. E. Holt 


(23°N, 68° W; IPL 1628/143620) 
(22°N, 73° W; MTVS 4297-7, 4297-15) 

A 700 km length of a southern channel in the Kasei Valiis is seen below. The flow direction 
of this channel is eastward into the Chryse Planitia. An area in the lower part of the photo 
(partly concealed by a dark circle produced in the Mariner television system) is shown in 
the high resolution mosaic at right (approximately 75 km wide I. A dendritic canyon sys- 
tem ap])ears to have developed along an angular fracture set by headward growth. Note the 
smooth-floored channels. Wind scour has etched relief features across the upper plateau 
level. The ejecta from the large crater form a distinct bench and are believed to be accentu- 
ated by the greater resistance of the ejecta blanket to wind erosion. — H. E. Holt 


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(29°S, 40°W; IPL 434/211030. 7462/40724. MTVS 4158-871 

The channel above is about 600 km long and 5 to 6 km wide. The lower reaches (top 
left) resemble the sinuous rilles of the Moon: the upper portion (top right) is more 
reminiscent of entrenched desert arroyos on Earth. The meandering and dendritic 
form of this channel is convincing evidence that a fluid once flowed on and eroded 
the planet's surface. — H. Masursky 

(20°S, 184°W; IPL 454/200454. 454/2031101 

This pair of low resolution photographs (left) shows a sinuous valley. Ma'adim Vallis, 
about 700 km long. The valley resembles shorter sinuous rilles on the Moon. The pre- 
vious existence of fluids is strongly implied by the widening and deepening toward 
the mouth of the channel and the multiple branched tributaries toward its head. Water 
could not exist in the present climate of Mars, so a different climate in the past is 
suggested. — H. Masursky 



(7°S, 151°W; MTVS 4294^20, 4294-16, 4294^12) 

Middle section of the Amazonis channel in Mangala Vallis where direction of flow is 
from right to left (south to north). The braided channel at right converges into a 
slightly sinuous main channel, 2 to 3 km wide, containing large bars "and streamlined 
islands along the streambanks. Several levels of stream terraces occur along the east 
bank (top side of channel) which indicate several stages of stream erosion. The stream 
terraces, bars, and braided channels suggest that the streambed was eroded by run- 
ning water where the quantity of stream flow fluctuated, perhaps becoming an inter- 
mittent stream. The individual frames cover an area about 30 by 40 km. — H. E. Holt 

(45°N, 116°W; MTVS 4182-96) 

The frame at right, about 60 km across, shows the eroded, undulating surface on a 
flank of Alba Patera. The fine textured dendritic pattern of deep gullies suggests 
erosion in unconsolidated material. An atmospheric source of water is suggested by 
the closeness of the channel heads to hill crests and by the presence of channels on 
both sides of elongated hills. Spotty distribution of such channels on the martian 
surface may have a climatic basis or merely be ascribable to obscuration of many 
gullies by wind erosion. — H. Masursky 











.#' 4 





f36°S, 248°W; MTVS 4244-27, 4244-31) 

The gullies on the inner wall of a 35 km wide impact crater, northeast of Hellas Pla- 
nitia, suggest erosion bv fluids. The origin of the sullies near the summit of the inner 
wall does not exclude melting ground ice as a source of fluid. The spur leading from 
the right rim mav be of volcanic origin, as suggested by the multiple sinuous linear 
features and by the conical peak (arrow I at the junction of the rim and spur. In the 
bottom right of the picture, a small steep volcanic cone (arrow) having a barely 
discernible summit crater is visible. It is part of an east-west array of similar small 
conical hills, that is perhaps a volcanic chain. The channel nearby is a tributary to a 
major 1300 km long channel which drains southwestward into Hellas. — D. B. Potter 

(9°S, 330°W; IPL 7243/111916) 

Gullies have eroded into the rims of old impact craters (below). Picture width is 
about 330 km. The patterns resemble gully svstems on moderate slopes in terrestrial 
deserts, and may have been formed by runoff of precipitation. — M. A. Sheldon 


(38°N, 330° W) 

This mosaic of low resolution photographs (above) shows the margin of a heavily 
cratered upland and the northern lowland that at the time was partially covered by 
clouds of the martian north polar hood. The edge of the highland is dissected by many 
sinuous and anastomosing channels that apparently are eroded into the highland. The 
channels shown here and those near Alba are at 45°N, the farthest north that channels 
have been perceived on the planet. The most abundant channels on Mars lie about 
10° south of the equator. — H. Masursky 

(6°N, 22°W) 

The channel in this mosaic (right) of an area associated with collapsed terrain 
descends north into the Chryse Planitia. The Chryse lowland is a low part of the 
martian surface and a part of the lowland that girdles the planet. The channel slopes 
northward about 5 meters per kilometer for 1200 km and is about 30 km wide. It 
may have been produced by release of water from chaotic terrain near its head by 
melting of permafrost. The channel is degraded (that is, some braided forms are vis- 
ible) and somewhat cratered. indicating an intermediate geologic age. — H. Masursky 



(8°S, 151°W; IPL 1691/160649) 

A complex of meandering valleys (left) cut through cratered terrain and debouch 
onto smooth plains in the upper part of the picture. As the valleys are traced down- 
slope, irregular dendritic furrows coalesce to form a few major channels. — T. A. Mutch 

(7°N, 45°W; IPL 1634^134231) 

On the edge of the Chryse Planitia, canyoned terrain (below) shows prominent chan- 
nels and rilles. The conspicuous light-dark boundary divides areas of unequal crater 
density. The lighter area has fewer craters; hence, it is probably a younger surface 
and it may be composed of a surface covering of fine particulate material that is being 
redeposited after erosion by the channels. — E. A. King, Jr. 


Fractures and Faults 

Fractures and faults are abundant on the martian 
surface. Faults extending radially from craters and iso- 
lated fractures thousands of kilometers long indicate the 
response of the martian crust to changing stress condi- 

Surface fractures associated with large shield vol- 
canoes and domes may result from the upwarping of the 
crust; possible later withdrawal of subsurface magma and 
concomitant collapse may produce faults. Radial and con- 
centric fractures are also present in crater fields, and are 
due presumably to the tremendous shock of impact and 
subsequent readjustment of the crust. 

The most common fracture-related feature is the 
graben: a valley formed when the area between two 
approximately parallel faults drops down relative to the 

areas on each side. Many grabens are radial to the Thar- 
sis volcanic field, suggesting that the broad uplift of the 
volcanic field and the attendant stretching produced many 
sets of faults and, subsequently, grabens. 

Fractures in volcanic regions commonly serve as 
weak or dilatant zones through which lava can escape to 
the surface, giving rise to an alignment of volcanoes or 
flow features. These alignments serve as an indication of 
now obscure fractures. Fracturing and faulting of the sur- 
face may also determine the trend of an escarpment of 
canyon. Such structural control is indicated by the occur- 
rence of linear escarpments, which commonly form inter- 
sections with other escarpments. — J. W. Allingham and 
J. S. King 


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(40°N, 108°W; IPL 1950/95214, 1950/130813) 

A system of graben (above), partly deflected around a volcanic complex, form a ring 
about 650 km across. Part of the system is buried under volcanic material. At least 
four parallel, narrow rilles (arrows) cut across the graben system. These rilles, or 
crater chains, probably are linear arrays of volcanic vents. The longest rille shown is 
more than 400 km in length. Note that some graben are arranged en echelon. — J. E. 

(38°N, 104°W; IPL 1428/223550) 

A detailed narrow angle view (left) of part of the graben system shown above (about 
60 by 80 km). Muhiple graben interrupt sinuous channels. Many fresh raised-rim 
craters are younger than the broken surface. A linear crater chain is present at upper 
right. — J. S. King 


(17°S, 110°W; IPL 1563/130615) 

The intersecting and offsetting relationships between faults (right) in this high resolu- 
tion view of part of the area shown below indicate the relative times and directions of 
movement of the faults. For example, graben A is olfset by fault B, which is in turn cut 
by graben C. Thus A must be the oldest of the three, and C is the youngest. Fault B is a 
strike-slip fault (a fault which has lateral rather than vertical displacement). The 
crater is 7 km in diameter. — J. E. Peterson and H. Masursky 

(15°S, 108°W: IPL 1108/144725) 

A system of subparallel fault lineaments trending northeast to southwest clearly define 
a family of graben (regions which have been down-dropped relative to surrounding 
terrain ) . A second less obvious and older system intersects these. The faulted area is 
smooth plain material with only a few relatively young craters superimposed. — J. S. 


(31°N,81°W; IPL 1434/180111) 

Complex system of graben near the Tharsis Montes (left) showing some graben offsetting 
older graben. The ejecta blankets of large craters partially cover some graben in the 
lower right of this picture, indicating an early age for much of the fracturing. Fluid flow 
in larger flat-bottomed graben may have modified walls and deepened valleys. Note the 
hanging valleys (arrows) on the sides of the deepest graben, which is 2 km wide. — J. W. 

(38°N, 140°W; MTVS 4256-60) 

A high resolution view (below) shows the gradual fading of the graben into the plain 
and possible evidence of fluvial modification of the graben. A second set of faint graben 
crosses the more prominent set. Note the tiny conical volcanoes (center) adjacent to 
the faults bounding the grabens. The area is about 45 km wide. — J. W. Allingham 


(21°S, 106°W; MTVS 4184r-90) 

This fractured plain is located east of Olympus Mens and north of Ascraeus Mons. 
The pattern is almost certainly controlled by a major set of north-south trending frac- 
tures, Claritas Fossae. The impact crater in the fractured plain is about 20 km in 
diameter. — J. E. Peterson 

(16°N, 142°W; IPL 497/191619) 

Grooved terrain forms a discontinuous aureole around Olympus Mons (right). It con- 
sists largely of closely spaced low ridges and intervening linear troughs that in high 
resolution pictures appear to have been wind scoured. The troughs almost surely repre- 
sent a complex array of fracture zones that are less resistant than the surrounding mate- 
rials to wind erosion. The origin of this terrain and its relation to Olympus Mons re- 
mains a puzzle. Some investigators have suggested that it represents an early outpouring 
of lava or ash from Olympus Mons that have since eroded back to the pronounced scarp 
that now surrounds this enormous volcanic edifice. The picture is about 365 km wide. 
—J. F. McCauley 


— ' ''-.* ■ 



Long, steep cliffs occur on the surface of Mars. They 
are from 1 to 4 km high, and range up to several hun- 
dred kilometers in length. 

Many escarpments have complex configurations and 
scars that suggest that some form of erosion has caused 
the scarp face to recede at the expense of the uplands. 
Numerous U-shaped chutes in the upper reaches of escarp- 
ments are similar to the scars left by debris avalanches 
on steep terrestrial slopes. Lumpy mounds of material 
below alcoves or gullies are indicative of debris slides 
or slow downhill movement. In regions bounding chaotic 
terrain, huge blocks that often retain their original flat 
tops have slumped downward and outward from the edges 
of escarpments. 

In contrast to the deeply embayed and scarred cliffs, 
there are also long escarpments with straight, sharp 
brinks and few scars. Because of this configuration, this 
form of structure is thought to follow faults or fractures, 
and to have undergone little recession of the face. 

In the polar and near-polar regions some scarps seem 
to be a product of erosion of layered material that mantle 
older, cratered terrain beneath. This observation suggests 
that Mars may have undergone alternating cycles of dep- 
osition and erosion, the latter attended by the develop- 
ing of retreating scarps. — R. P. Sharp 


(15°N, 130°W; MTVS 4265-48) 

The southeastern portion of the Olympus Mens escarpment (above) shows a well de- 
fined base and generally a sharp rim with apparent slump scarps and terraces. The 
fluted, steep, upper part is partially covered by huge landslides or lava flows. The 
escarpment varies from 1 to 3 km in height. Residual block-like mesas indicate the 
remnants of a higher terraced surface on the flank of the volcano. — D. B. Potter 

(18°N, 134°W) 

The great escarpment (left) around the base of Olympus Mons, approximately 1500 
km long, resembles a wave-eroded seacliff on a terrestrial volcanic island, but is not so 
easily explained as there are no martian seas. The escarpment appears sharp over 
more than half of its length; the remainder appears subdued. In a few places the 
scarp is absent, probably covered by lava flows or huge landslides. The origin of the 
escarpment is uncertain, but probably involves a combination of such processes as 
mass wjisting and eolian erosion. — J. E. Peterson 


(2°N, 111°W; MTVS 4229-51) 

A detailed view of the northeast flank of Pavonis Mons (below I shows many features 
characteristic of collapse between fractures which are called graben. The graben trend 
northeastward, parallel to the Tharsis Montes. Rock chutes and ridges have been modi- 
fied by wind action. — J. W. Allingham 

(9°S, 69°W; IPL 7224/160459) 

The elongate flat-topped highland area (right) is comparable to the mesas common in 
arid regions on Earth. This mesa, rising 2 to 3 km above the floor of the huge Coprates 
Chasma, is about 400 km long and 150 km wide and connects (not shown) to an ex- 
tensive plateau area north of the canyon. Sculpturing on the steep slopes of the mesa 
indicates downslope movement of material by landsliding, leaving the characteristic 
U-shaped chutes. The apparent absence of landslide deposits below the chutes suggests 
their removal by wind or running water. The top of the mesa is extensively transected 
by faults, some of which occur in facing pairs so as to produce long, narrow troughs or 
graben.— G. E. McGiU 




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(13°S, 71°W; MTVS 4195-33) 

This arcuate escarpment, several kilometers high, is a portion of the south wall of 
Valles Marineris at one of its widest points, Melas Chasma. Erosion by mass wasting 
appears to be the dominant process involved in the escarpment retreat. Debris ava- 
lanche chutes are abundant along most of the scarp. Note the long ridge extending 
about 80 km into the canyon. — J. E. Peterson 

(12°S, 50°W; IPL 7464/235907) 

The sharp rim edge along the northern edge of the equatorial plateau (above) indicates that 
resistant rocks underlie the plateau. The escarpment is 1 to 2 km high and alternating resist- 
ant and nonresistant rock layers are exposed on the cliff faces. These may be alternating 
lava flows and pyroclastic rocks as these exposures are not too far from the great volcanoes 
that may have acted as the source for the rocks. The rock layers may be from 100 to 200 m 
thick. The few impact craters on the surface of the plateau imply that it is geologically a 
young surface. — H. Masursky 

(5°S, 85°W; MTVS 4275-36) 

Detail of Valles Marineris wall and edges of the plateau. Note the apparent raised rim of the 
plateau and occurrence of bedded outcrops just below the rim. The picture is about 63 km 
wide and the escarpment is several kilometers high. — D. B. Potter 


(41°S, 258°W: IPL 1445/105008) 

This isolated bold mountain remnant on the plains at the east edge of Hellas Planitia 
is approximately 35 km wide. Its sharp ridges and spurs have a branching pattern 
indicating equal erosive attack from all sides. The steep upper slopes show mass wast- 
ing chutes and narrow tongues of material suggest some form of mass movement. 
Broader tongues of material occur along the western base. Around the base of the 
mountain is a wide apron sloping gently away from the mountain. This suggests slow 
mass movement of granular material over a long period of time. — D. B. Potter 



Fretted and 
Chaotic Terrains 

Fretted and chaotic terrains are lowland topographic 
forms on the martian surface which may be in part the 
product of related genetic agents. Fretted terrain is char- 
acterized by smooth, flat lowland areas with many flat- 
topped buttes and mesas resembling those in the western 
United States. Chaotic terrain exhibits rough floor topog- 
raphy of jumbled large, angular blocks. Both terrains are 
separated from cratered upland areas by escarpments 
having complex configurations. 

A striking characteristic of fretted terrain is its ir- 
regular border pattern. The steep escarpment is smoothly 
sloping and free of slump blocks and typically traces a 
ragged course with deep embayments, projecting head- 
lands, and numerous shallow scallops. The lowland floor 
of the fretted terrain is generally smooth, showing only 
a few scattered craters and low swells and swales. 

Some areas of chaotic terrain are sharply bounded 
by an abrupt escarpment of irregular configuration, while 
other boundaries exhibit a transition from slightly frac- 
tured upland through a highly fractured zone to a jumble 
of irregular blocks. The vertical relief of escarpments 
seems to range between 1 km and 3 km. They are higher 
than most escarpments bounding areas of fretted terrain. 

The most distinctive feature of chaotic terrain is the 
rough-floor topography consisting of an irregular jumble 
of angular blocks up to several kilometers wide and tens 
of kilometers long, many bearing remnants of the rela- 
tively smooth upland surface on their tops. At some sites, 
the shape of the blocks appears to be controlled by inter- 
secting sets of fractures resulting in blocks of almost 

equal dimensions. After formation, the blocks appear to 
undergo continuing breakdown and reduction in size, 
eventually being completely destroyed and leaving a flat, 
smooth floor similar to that of fretted terrain. 

Fretted terrain, clearly developed at the expense of 
older cratered uplands, appears to be among the youngest 
of the martian landforms. The smooth floors of most areas 
of fretted terrain are only sparsely cratered, mostly by 
small, new craters. Chaotic terrain is judged to be equally 
youthful on essentially the same basis. Closely adjacent 
areas of fretted and chaotic terrain cannot be too differ- 
ent in age. However, the seeming paucity of craters within 
areas of chaotic terrain may be a result of difficulty in 
recognizing small craters within the chaos of jumbled 

Subsidence and slumping having played a part in the 
development of chaotic terrain. Since these are usually 
initiated by the removal of subsurface material, the prob- 
lem of the origin of chaotic terrain becomes one of identi- 
fying the material removed and the process, or processes, 
which accomplished the removal. 

The development of fretted terrain is thought to be 
initiated by some structural break in the old cratered 
uplands. Once an escarpment is formed, it recedes by 
some type of undermining or sapping mechanism. The 
erosional removal of debris, perhaps by the wind, leaves 
a smooth, flat floor and isolates island-like buttes and 
mesas. The bounding slopes of these outliers also recede, 
reducing them in size until they disappear entirely. — 
R. P. Sharp 


^^^^^^^^^^^^^Kf^^^^^^- ^': W^*-m 



.^m^ V 

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(44°N, 330°W; IPL 1417/224259) 

A close-up view of erosional outliers in an area of fretted terrain. The height of the 
prominent features is at least 1 to 1.5 km. The undulating plain shows numerous 
swells and swales. The young, raised rim crater is about 3 km across. — R. P. Sharp 

(43°N, 313°W; IPL 1651/154245) 

In this fretted terrain (left) at mid-latitude in the northern hemisphere, a relatively 
smooth lowland is separated from the old cratered upland by abrupt cliffs at least 1 to 
2 km high. Mesa-like remnants and flat-floored chasms penetrating far into the upland 
are characteristic. This terrain is regarded to be a product of cliff recession caused by 
an undermining process operating at the cliff base. Material shed by the cliffs has 
been removed, probably either by fluvial transport, under different climatic condi- 
tions, or by eolian deflation. — R. P. Sharp 


(3°N,37°W; IPL 7350/165312) 

Association of chaotic terrain (upper right, facing page) with flat-floored steep-walled fea- 
tures that are characteristic of fretted terrain suggests some common genetic influences. Note 
the arcuate slump blocks at the lower edge of the chaotic area (arrow). The flat-floored 
chasm leading to the left may have been modified and widened by the recession of walls as a 
result of undermining or it may represent a channel carved or modified by a huge flood which 
burst forth from the area of chaotic terrain. — R. P. Sharp 

(1°S, 20°W; IPL 7059/162910) 
(4°S, 20°W; IPL 1411/212107) 

Views of chaotic terrain (below), formed by the collapse of an old crater upland when its 
underlying support was withdrawn. In the top photo, a broad, seemingly scoured channel can 
be seen emerging from the chaotic area in the upper right corner; the photo at bottom shows 
a close-up view of the broken blocks in the right central part of the top photo. — R. P. Sharp 




* ti 





••!!!!fHi»w!";:^ B fe<ij i 

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(3°N, 28°W; MTVS 4203-60) 

This moderately cratered surface extends over several thousand square kilometers. 
Three areas shown in this photo consist of complex mosaics of broken surfaces ranging 
from over 15 km across down to the resolution limit of several hundred meters. The 
massively fractured and slumped chaotic terrain generally lies below the level of the 
surrounding older surface. Large channels originate in the chaotic terrain area and 
extend many hundreds of kilometers northward. The chaotic terrain and channels may 

have resulted from removal of materials in the subsurface with consequent collapse 
of overlying strata. Perhaps some form of ground ice melted, and the resulting liquid 
drained away forming the large channels. However, physical/chemical processes 
needed to produce such large quantities of ground ice, and later to supply large 
amounts of local heat to melt the ice in a very short time span, are not recognized 
topographic/geologic processes. — H. E. Holt 



Large circular basins like those which enclose Hellas 
Planitia, Argyre Planitia, and Isidis Planitia are the old- 
est recognizable structures on Mars. Several of the basins 
display remnants of concentric rims and radial fractures, 
and appear similar to lunar multi-ring circular basins. 
These great basins are believed to have been formed by 
impact during planetary accretion, and thus may be 
classed as ancient super-craters. 

Although numerous martian craters are of volcanic 
origin, the great majority of them are probably the result 
of impact. The oldest heavily cratered terrain is saturated 
with large craters having diameters greater than 20 km. 
Such terrain occurs preponderantly in the equatorial zone 
and the southern hemisphere, including the polar area, 
which appears to contain many subdued craters. Small 
craters are rare or lacking in the polar regions. 

Most large martian craters have been modified by 
subsequent impact, blanketing, and eolian processes. 
Many craters are subdued, with extensive wall slumping 
and infilling. They are shallow, have flat floors, and 

usually lack central peaks. These characteristics probably 
result from deposition of material, perhaps volcanic, after 
the craters formed. 

Several aspects of martian craters are noteworthy. 
Many of them are doublets, nearly tangential, and about 
the same size. These could have been formed by internal 
processes such as the collapse of volcanic structures, or 
by impacting masses that broke apart before striking the 
surface. Some impact craters appear to stand on plateaus 
or pedestals. This effect might have been caused if ejecta 
had possessed a greater resistance to erosion than did the 
general terrain material. Other martian craters may have 
been caused by low-angle impact, as suggested by elon- 
gate form and bilateral ejecta patterns. 

In general, the cratering histories of Mars and the 
Moon may have been similar. But differences in planetary 
size and gravitational attraction, as well as the presence 
of an atmosphere and extensive deposition of filling 
material, have led to certain characteristic differences in 
crater morphology. — D. E. Wilhelms 


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(53°S,81°W: IPL 406/1927722 

The double-ringed basin Lowell (left). 200 km in diameter, is most probably of impact 
origin. Sharp-textured ejecta attest to its relatively recent formation. In comparison to 
craters having single rim crests and multiringed circular basins, this crater is inter- 
mediate in diameter and in number of rings. It shows what many older, degraded 
features once looked like. — D. E. Wilhelms 

... JH 









(46°S, 44°W; MTVS 4139-9) 

This smooth plains area is the northern half of Argyre Planitia and is about 800 km 
across. Careful study of this and adjoining photographs reveals concentric rings of 
high, rugged terrain around the plains, similar to the multiringed circular basins seen 
on the Moon. — D. E. Wilhelms 


(16°S, 350°W; MTVS 4287-24) 

Typical cratered terrain (right) has both old, smooth-rimmed craters, and younger, 
sharp-rimmed ones. The large one at the top is 165 km across, with a conspicuously 
flat floor and slumped walls. Note the small doublet craters at lower left with central 
peaks. — M. Gipson, Jr. 

(5°N, 250°W; MTVS 4194-60) 

This cratered terrain (below) also shows lineated features made up of plateaus and 
troughs. They align radially with the Isidis basin, which is outside this image toward 
the northwest. — D. E. Wilhelms 





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(38°N, 343°W; MTVS 4210-66) 

The sharply defined little crater in the center of the photo above is relatively young, as 
attested to by its bowl-shaped floor, raised rim, and well-preserved, ray-like ejecta 
blanket. Its diameter is 3 km. — J. W. Allingham 

{23°N, 290°W; MTVS 4183-90) 

A combination of densely and moderately cratered terrain (left) also includes popula- 
tions of old craters and those of moderate age. Scale of this photo is about 450 km 
across the top. Two parallel troughs occur at the bottom. — D. E. Wilhelms 


(3°N, 304°W; IPL 1764/235027) 

A continuum of crater types (above) is revealed in this picture, ranging from the 
subdued, knicked one at bottom to the sharp-edged, relatively young one at top, with 
its ejecta blanket and small central peak. — D. E. Wilhelms 

(38°N, 335°W; MTVS 4212-66) 

Impact origin (right) is probable for this 15 km crater. The hummocky texture of its 
ejecta blanket suggests that little erosion has occurred since its formation. Neverthe- 
less sufficient time has elapsed for several kilometer-sized craters to have been produced 
in the ejecta. Many small craters can be detected in this picture down to the limits of 
resolution of several hundred meters. — D. E. Gault 


Wind -Shaped Features 

Mariner 9 convincingly demonstrated that wind is 
the dominant agent of erosion and sedimentation on 
Mars. In addition to the great dust storm of 1971, a wide 
variety of features that can be ascribed to wind activity 
were found. Unlike craters on the Moon (which lacks an 
atmosphere) the craters on Mars show the effects of both 
eolian erosion and deposition. Most craters tend to have 
flatter floors and less distinguishable surrounding ejecta 
blankets. Others appear to have been once buried and are 
now being exhumed by wind action. The equatorial re- 
gions of Mars appear to be areas where wind erosion 
predominates over deposition. This can be seen in nu- 
merous examples of streamlined canoe-shaped hills, fluted 
cliff faces, along with multitudinous parallel grooves on 
the surface of the flat plains. Similar appearing features 
are found only in the most rainless and wind-swept 
deserts of the Earth. (See "Similarities: Mars, Earth, and 
Moon.") The midlatitude and polar regions appear on 
the other hand to be areas where deposition of fine wind- 
blown material predominates. These deposits produce 
vast almost featureless plains that bury earlier craters 
and volcanic flows. 

The changes in the surface markings of Mars have 
been a puzzle to telescopic observers for generations. 
Many elaborate hypotheses have been invented to explain 
these in terms of vegetation, volcanic activity, or chemi- 
cal changes. Mariner 9 has shown that almost all the 

surface markings can be explained by wind activity. 
Many of the abundant light and dark streaks are asso- 
ciated with craters and other topographic features. These 
frequently merge into broader mottled patterns that at the 
telescope would have appeared to be continuous dark 

During the Mariner 9 mission some of the streaks 
actually changed shape and position indicating that they 
are superficial. (See "Changing Features.") One explana- 
tion is that the light areas are zones of deposition of re- 
cent dust and fine sand. The dark areas are zones of 
somewhat coarser and darker material where dust and 
sand have been removed or were simply not deposited. 
The same situation prevails in terrestrial deserts in the 
lees of topographic obstacles. Some of the dark areas in 
the floors of craters, on the other hand, proved to be 
large dune fields, further convincing evidence for the 
dynamic role of the wind on Mars. We now know that 
the wind is currently sculpturing the surface of Mars, re- 
moving silt and clay from some areas and redepositing it 
in others. As will be seen in other sections of this book, 
fluvial and glacial activity also have taken place. Thus 
primarily because of its atmosphere, however tenuous, 
Mars as had been surmised from the early telescopic 
observations to more closely resemble the Earth than 
does the Moon. — J. F. McCauley 


(11°N, 283°W; MTVS 4186-69) 

Dark and white streaks on the slopes of Syrtis Major Planitia. Viewed telescopically, 
the surface of Syrtis Major Planitia is dark, and has an eastern variable edge, whose 
lateral variation is enhanced by seasonal albedo changes. On Mariner 9 images, this 
dark region is resolved into a series of sub-parallel dark and white streaks, which ex- 
tend several hundred kilometers. In this high resolution picture (40 km wide) taken 
by the Mariner 9 camera on January 30, 1972, a few weeks after the end of the 1971 
dust storm, dark streaks extend from craters and unresolved point obstacles which 
protrude above an otherwise smooth surface. The small streaks extend as many as 50 
crater diameters beyond the crater obstacle, and invariably flare in an easterly direc- 
tion. The wider and nearly continuous dark streaks in the right of the picture extend 
from a large crater located outside the image. These dark streaks resemble the dark 
wind shadows formed in the lee of obstacles, particularly downwind from the slip 
face of transverse dunes (barchans) in terrestrial deserts, where turbulent eddies in 
back-sweeping motion remove light-toned eolian sand from a darker (and coarser) 
desert pavement. At the same time, sand saltates and creeps away downwind from the 
barchan horns. Both processes are concomitant on the Earth, and so they are on Mars. 
In support of this assertion is the digitate or serrated outline along the edge of the 
widest white streak in the image. The white "teeth" along this irregular contact between 
white and dark streaks point the same way as the flares in the dark streaks. They re- 
semble the front of a sand sheet advancing over a barren terrestrial surface. The over- 
all pattern of light and dark markings is confidently ascribed to the work of unidirec- 
tional, westerly winds. — M. J. Grolier 



..a •* 






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i^ * 


(47°S,330''W: IPL 267/220940 ) 

(47°S, 330°W: MTVS 4228-15. 4264-15, 4264-19) 

A dark zone in the floor of a crater near Hellesponti Montes is seen in the low resolution 
photo above. Similar appearing dark splotches appear in the floors of many Mars craters. The 
high resolution photomosaic at left reveals that the dark zone is an elliptical dune field 
about 130 by 65 km in size. The dune field consists of series of subparallel ridges, 1 to 2 
km apart, that closely resemble terrestrial transverse dunes. Many of the ridges appear to 
have rounded crests with similar slopes on either side. This suggests that although the wind 
here generally blows at right angles to the transverse ridges it may intermittently reverse its 
direction so as to even out the slopes on the windward and lee sides of the dunes. The un- 
usually dark appearance on what appears to be the more windward side of the dunes may be 
concentrations of dark heavy minerals such as ilmenite and small dark lithic fragments. On 
Mars, concentrations of such heavy minerals may have become preferentially trapped in 
crater floors because of nind action. — J. F. McCaulev 

(38°N, 260°W; IPL 1433/210342) 
(5°N, 152°W; MTVS 4294^28) 

Differential erosion of two different types around probable impact craters. Top right, a sharp- 
rimmed, 20 km crater is encompassed by a radially and concentrically fractured rim unlike 
that seen around any lunar crater. Similar features are known to occur in the bedrock be- 
neath the ejecta of terrestrial impact craters. This suggests that wind action has completely 
stripped away the original ejecta deposit exposing the shock deformed pre-crater surface. In 
contrast "pedestal" craters 1 to 2 km across, also different from any lunar crater, are seen 
below right. They are surrounded by sharp serrate scarps that coincide approximately with 
the boundary of what would be the continuous ejecta blanket of an impact crater. In this case 
the rubbly ejecta appear to have operated as a temporary "armor" acting to protect the sur- 
face on which it lies while the less resistant surrounding plain was being lowered by wind 
erosion. — J. F. McCauley 

(71°S, 217°W; MTVS 4264-19) 

Highlighted by frost, probable eolian features are seen in the specially processed high reso- 
lution photograph below. The features are most likely wind blown dunes of martian sand and 
dust. These dune-like features occur in craters located along the margin of the layered ter- 
rain in the south polar region. The features appear to be confined by the closed topography 
of the craters. The dunes are in a crater partly buried by layered terrain. Individual dunes 
are approximately 3 km apart. The area shown is about 40 km wide. — L. A. Soderblom 



(87°S, 273°W; MTVS 4248-12) 

Flutes and linear grooves (right) in the layered terrain exposed near Australis Chasma, 
south polar region. There is no polar cap shown here. Bedding is enhanced by the 
contrast between the light and dark layers exposed in steep bluffs, and the sides of the 
prominent ridge in the eastern part of the area. The short, finely structured striations 
in the bluffs stand out in contrast against the smooth surfaces of terraces and hollows, 
which are perhaps mantled with wind-blown material. Striations in the bluffs and 
wider flutes on gentler slopes are parallel, and best developed on south-facing slopes. 
A hill in the center of the image is grooved at one end, and beveled at the other end, 
much like some terrestrial yardangs are. The erosional pattern suggests that wind ero- 
sion, together with possible melting and sublimation of the underlying material, are 
the processes modifying this polar landscape. Scouring here is accomplished by winds 
originating near the South Pole (outside the imaged area) . — M. J. Grolier 

(74°S, 7°W; MTVS 4270-24) 

Irregular pits and depressions near the south polar region are shown below in this 
high resolution photo. These depressions are generally characterized by flat floors and 
rather smooth walls. They are very similar to terrestrial deflation hoUows formed by 
the plucking and scouring action of the wind. These landforms, like the other 
probable martian wind features described in this section, are many times larger 
than their terrestrial counterparts, the largest known of which are in the desert of 
north central China. Picture width is about 75 km. — J. W. AUingham 






>>**!;*« ■■ 

-. ■■. *#c 

(5°N, 146°W; IPL 1596/212535) 

Wind etched semi-parallel grooves occur on probable bedrock in the relatively smooth. 
uncratered plains of southern Amazonis. The width of the picture is about 40 km so 
that the alternating, streamlined ridges and grooves are typically about 200 m in width 
and tens of kilometers long. This pattern is probably controlled in part by bedrock 
fractures. Similar parallel scouring of homogeneous materials does occur, however, in 
the flat open parts of terrestrial deserts that are characterized by strong, almost uni- 
directional prevailing winds. On Earth similar appearing wind scour features are, 
however, many times smaller in size. Although the martian atmosphere is one hun- 
dred times less dense than that of the Earth, the wind velocities may be on the order 
of 200 to 300 km per hour. Thus the kinetic energies of particles moved by the wind 
will be many times greater and the erosional effect of sand blasting a far more im- 
portant geologic process than on Earth. — J. F. McCauley 




Changing Features 

Telescopic observations of Mars show that its appear- 
ance changes with the seasons. As the polar cap recedes 
toward the summer pole, a progressive contrast enhance- 
ment between the bright and dark areas takes place. This 
seasonal change starts in spring at the edge of the reced- 
ing cap and proceeds toward the equator, and is referred 
to as the wave of darkening. As observations of Mars 
from Earth are very difficult — the attainable resolution 
is about 60 km — many theories have been offered. The 
darkening was once thought to represent martian vegeta- 
tion responding to water vapor released into the atmos- 
phere by the receding polar cap. 

Such changes can now be explained in terms of 
windblown dust. According to the simplest version of 
such a model, a large dust storm occurs each martian 
year soon after perihelion and covers most of the planet's 
dark albedo markings with a thin layer of fine, bright 
dust. Because of local conditions, such as topography, 
subsequent seasonal winds will scour these bright parti- 
cles more efficiently from certain regions than from 
others. Those regions which are efficiently swept will re- 
appear as dark features first. 

Many of the classical variable regions of Mars, for 
example Promethei Sinus, were observed by Mariner 9 
to be cratered terrains liberally sprinkled with dark amor- 
phous spots which we may call "splotches." These 
splotches are closely related to winds, since when they 

occur in craters they are usually found tucked up against 
a crater wall on the downwind side of the crater. 

High resolution photography provides other proofs 
of the connection between splotches and Martian winds. 
Some splotches show scalloped edges, a characteristic 
sign of wind erosion. Sequential observations have shown 
splotches to be highly variable with time. In many clas- 
sical variable regions, such as Syrtis Major Planitia, the 
albedo boundaries seen from Earth are determined by 
a superposition of bright and dark streaks. Syrtis Major 
— perhaps the most famous dark region on Mars — has 
long. dark, wind-related, curved streaks trailing from its 
craters. Most of the eastern boundary of Syrtis Major is 
defined by such streaks, and these wind-related streaks 
change with time. 

During the Mariner 9 mission the dark streaks in 
Syrtis Major grew. The simplest explanation is that winds 
were sweeping up the bright dust that was deposited over 
most of the dark material in Syrtis Major by the 1971 

Mariner 9 has confirmed that true changes occur on 
Mars. These changes are best explained in terms of wind- 
blown dust, and do not require a biological explanation. 
Of course, this does not demonstrate that life does not 
exist on Mars; the only way to settle that argument is to 
land on the surface and look. — C. Sagan, J. Veverka, 
P. L. Fox, and L. Quam 


(70°S, 259° W; MTVS 4211-9) 

A low resolution view of Promethei Sinus (above) shows an extensively splotched, cratered 
region near the south pole of Mars. This picture is about 450 km across. Variations in the 
appearance of this region have been reported by telescopic observers. Note that the dark 
crater splotches tend to lie on the downwind side of crater floors. A small crater near upper 
center is shown at high resolution on the facing page. — C. Sagan 

(70°S, 253°W; IPL 1418/143014) 

This high resolution view of a region in Promethei Sinus was studied for surface variations 
during the Mariner 9 mission. The scalloped appearance of the albedo boundaries is charac- 
teristic of eolian phenomena — the inferred wind direction being at right angles to the scal- 
loped edge. Variations in the crater splotch and in the leaf-shaped albedo marking just left 
of crater, most likely due to the removal of mobile material by w inds, are shown on the fol- 
lowing pages. — C. Sagan 













(70°S, 253° W) 

The dark pattern changed from rev 99 (top left) to rev 126 (top center) ; rev 126 to 
rev 179 (middle row) ; and rev 179 to rev 181 (bottom row) . After the images from each 
set (left and center) were similarly scaled and projected, then the two pictures were 
differenced, picture element by picture element. Images on the right show the differ- 
ences (Stanford AIL Picture Product STN 9167:050609, 10, 11). Thus, it is possible 
to see the changes that had occurred between successive revolutions over the same 
area. Each frame is about 30 km across. — C. Sagan 



(70°S, 253°W) 

These comparisons show changes in the crater splotch in Promethei Sinus. Differences 
between preceding views are shown at the right in each row ; the left and center views 
are from (top row) revs 126 and 179; (middle row) revs 179 and 181; and (bottom 
row) revs 181 and 220 (Stanford AIL Picture Product STN 0173:061109, 10, 11). 
Since lighting and viewing conditions varied slightly, changes in shadows caused by 
topography cannot be successfully canceled out. Each frame is about 30 km across. 
— C. Sagan 

Il " 




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^ : - 






.•( ■« uf* 

(10°N, 283° W; MTVS 4186-69) 

The eastern edge of the classical albedo feature, Syrtis Major, is outlined (left) by a 
concentration of variable dark streaks. The patchy, discontinuous character of these 
streaks is unique on Mars. This characteristic, as well as the tendency for the streaks 
to shed off tangentially in a common direction from topographic protuberances such 
as crater walls, suggests that they are produced by eolian erosion of extensive, but 
very thin, deposits of bright albedo material, resulting in the exposure of dark, under- 
lying, wind-resistant formations. This low resolution view is about 370 km across. 
— C. Sagan 

(10°N, 283°W) 

The gradual darkening of Syrtis Major (above) after the 1971 dust storm is revealed 
by Mariner 9 photography. The effect of the storm may have been to cover the area 
with a thin layer of bright dust. Subsequent winds, blowing predominantly west to 
east (the direction of the dark tails), scoured off this material, especially in regions 
where wind speeds are intensified by topography. The views are from rev 155 and 
rev 233; the image at right shows the difference of the two (Stanford AIL Picture 
Product STN 0164:041506). The area is about 130 km across, and corresponds to the 
lower left portion of the photograph on the facing page. — C. Sagan 


(23°S, 241°W; IPL 311/210101) 

A low resolution view (right), about 400 km across, of a region in the Hesperia Planum 
shows numerous parallel, light streaks associated with craters. A credible explanation of 
such an array of long parallel streaks emanating from craters is that fine, bright dust, trans- 
ported into craters in the waning stages of the dust storm, was subsequently blown out by 
high velocity winds having a prevailing direction. In any case the streaks must point down- 
wind, and are natural wind direction indicators. — C. Sagan 

(10°S, 107°W; IPL 1108/150842) 

An area in Tharsis (below), about 160 km across, characterized by an assortment of bright 
streaks showing strong evidence of an eolian streak stratigraphy. No variation in the config- 
uration of these streaks was observed during the Mariner 9 mission. — C. Sagan 




■.Ifii-.a*. .mi 




(9°N, 191°W; IPL 1612/173205) 

A region near Cerberus (above), about 245 km across. The prominent dark streak is 
probably depositional in character. An upwind crater can be interpreted as the source 
of the dark material which, carried downwind, produced the dark tail. In the process, 
a part of the rim of the smaller crater appears to have been covered by dark material, 
but there is also a shadow zone behind the smaller crater where no deposition occurred. 
There is a similar wind shadow behind a hillock near the lower right edge of the longer 
tail. — C. Sagan 

(34°S, 62°W; IPL 1934/171817) 

Dark crater streaks (left) stand out in this view of a region in Bosporos. This area is 
about 330 km across. Some of these dark streaks are more than 50 km long, yet 
remain very narrow throughout their length. Their common direction is that of the 
prevalent winds in the area. — C. Sagan 



Extensive Plains 

Extensive, flat to gently undulating plains occur over 
vast areas on Mars. They are most prevalent in the mid- 
northern latitudes, and also occupy extensive areas periph- 
eral to the poles and the floors of large circular basins. 
These plains are nearly devoid of relief except near their 
margins where they grade into other types of terrain. The 
plains commonly embay or surround the adjacent terrain 
as if the latter were being inundated. 

Most of the plains on Mars — like the maria on the 
Moon — probably formed when huge volumes of fluid 
lava erupted, spread outward, and buried the preexisting 
terrain. In some areas the plains are marked by incon- 
spicuous lobate scarps and subdued sinuous channels. 
The scarps are similar in appearance to the fronts of 
many lava flows on Earth, and the channels to lava chan- 
nels or collapsed lava tubes. 

The martian plains have been extensively modified 
by the action of violent wind storms. The plains sur- 
rounding both polar regions are interpreted as a mantle 
of windblown debris. A spiral pattern of light and dark 
markings along the margins of the circumpolar plains 
suggests erosion and deposition by strong winds originat- 
ing at the poles and blowing toward the equator. As the 
distance from the poles increases, the mantle of wind- 
blown debris thins. The plains deposits cover cratered 
terrain in the southern mid-latitudes and, except for the 
large circular basins, the plains are not well developed. 

In the northern mid-latitudes, however, volcanic plains 
are extensively mantled. Some scattered flat plains at 
higher elevations in the equatorial regions are more 
densely cratered than are the plains in the basins and 
mid-latitudes, and may represent an earlier plains-form- 
ing episode in Mars history. 

The scattered large circular basins and other low- 
lying areas probably act as relatively permanent sedi- 
ment traps for wind-borne debris. The polar winds trans- 
port some material for entrapment and equatorial winds 
carry more material to accumulate in the sediment traps. 
In most pictures of the large basins of Hellas and Ama- 
zonis, the plains appear to be nearly devoid of craters 
and other features having topographic relief. Because of 
their seemingly low crater density, the plains in these 
basins have been assumed to be covered with some of the 
youngest deposits on Mars. 

Some plains materials, especially those along mar- 
gins near the mouths of channels, may have originated 
as fluvial deposits. A good example is the Chryse region 
where the great Valles Marineris system empties into the 
basin of Chryse Planitia. If the Valles Marineris and 
other systems of canyons and channels have been formed 
by fluvial action, then the plains at the mouths of the 
channels would be analogous to terrestrial alluvial fans, 
although on a very much larger scale. — E. C. Morris 


JAN 23, 1972 

L« '*W- 

;-!>, . 

MAR 11, 1972 



(14°S, 185°W; MTVS 4211-42) 

Almost featureless even to the high resolution camera, this plain (above) in the center of 
the Amazonis basin shows only a few small, widely spaced craters. The largest crater in upper 
center is approximately 2 km in diameter and the smallest crater that can be seen is approxi- 
mately 500 m in diameter. Except for three prominent craters, all craters appear very sub- 
dued possibly because of a haze layer or blowing dust close to the ground, or because the 
craters are partially or almost completely buried by thick wind-deposited sediments. — E. C. 

(46°S, 307°W; MTVS 4167-9) 
(46°S, 305°W; IPL 1351/192301) 

Smoothness can be the actual nature of the surface, or can be in the eye of the beholder, or 
in weather, or in the imaging system. Picture taken on January 23, 1972, shows an area of 
the Hellas Planitia (80 km) with no surface detail. (The faint circular outlines are arti- 
facts in the imaging system.) Picture taken on March 11 shows ridges, craters, and other 
detail of the same area, indicating that a dust storm was active at the time the first picture 
was taken. Historically the Hellas basin has been the site of large dust storms as viewed 
telescopically and may have almost semi-permanent obscuration of its floor by blowing dust. 
It was probably fortuitous that the picture taken on March 11 was at a time when the at- 
mosphere had cleared sufficiently to record the detail seen in the picture. Some dust still may 
have been in the local atmosphere since details are somewhat subdued. — J. E. Peterson 


(1°N, 147°W; MTVS 4174-57) 

Most of the plains on Mars probably were formed when huge volumes of fluid lava 
erupted onto the surface and buried the pre-existing terrain. These volcanic plains 
were subsequently buried under a mantle of wind deposited sediments. The fluted 
and lobate escarpment in the center of the picture at right was probably the terminal 
end of an old lava flow that has been stripped of its cover of sand and dust and eroded 
by the action of the violent winds. This erosive process has also etched and enlarged 
fracture patterns on top of the flow. — E. C. Morris 

(17°S, 136°W; MTVS 4179-30) 

Subdued escarpments (below) may be seen along the margins of some plains. They 
may be the terminal fronts of ancient lava flows, partly mantled by eolian deposits. 
Similar lobate escarpments are seen on the lunar maria. — E. C. Morris 


»Af -• - V lA. 



M M 


FiWi 1*1 








:■' ^ 

(51°N, 263°W; MTVS 4289-48) 

In northern latitudes the plains are characterized by numerous small craters, hills and 
knobs, and patchy light and dark markings. The dark markings appear to create pat- 
terns, almost polygonal in form, similar to patterned terrain in the Earth's polar areas. 
— E. C. Morris 



Polar Regions 

The martian polar regions are of special interest be- 
cause they contain two unusual and unique terrains, 
pitted plains and layered deposits. These two regional 
units are superposed on ancient densely cratered terrains 
in the south polar region and on relatively lightly cra- 
tered plains in the north. The uniqueness of the etch- 
pitted plains and layered terrains to the polar regions 
leads to the inference that their formation must involve 
frozen CO2 or H2O. 

The pitted or etched plains vary widely in appear- 
ance, but typically are characterized by a level surface 
indented by numerous pits or irregular depressions. In 
places the pitted plains are being eroded, exposing the 
underlying cratered terrain. The processes of burial and 
exhumation do not appear to have modified the under- 
lying terrain significantly. 

The layered terrain is characterized by narrow, 
evenly spaced bands interpreted to be ledges of outcrop- 
ping strata of nearly horizontal strata. The strata are 
from 20 m to 50 m high, and a sequence composed of 
more than 100 such units has been measured near the 
south polar region. The absence of craters in layered 

terrain suggests that either it is one of the youngest units 
on Mars or the most actively eroded. 

The polar ice caps lie upon the laminated terrain. 
Each pole has a permanent cap composed of frozen car- 
bon dioxide or water and a thin ephemeral layer of car- 
bon dioxide, which forms poleward of the 60° parallels 
each winter and evaporates each summer. 

The cratered plains are obviously the oldest units in 
the polar regions because they are overlain by all of the 
other units. The pitted plains are believed to have been 
deposited next. Their origin is problematical, but the 
most convincing explanation seems to be that they repre- 
sent a thick blanket of fine dust which has settled out of 
the atmosphere at the poles perhaps trapped by water 
and carbon dioxide ices. Locally this blanket has since 
been eroded by the wind, producing pits. Layered ter- 
rain, the youngest unit, occurs within about 15° of the 
poles. The obvious stratification within the laminated ter- 
rain may have been caused by periodic changes in at- 
mospheric conditions while the material was being 
deposited. — L. A. Soderblom 




i \ 


Airbrush renditions give a generalized overview of the north polar region and the 
south polar region. They clearly show the residual ice caps and the distributions of the 
various types, of terrain. The south polar region seems much more heavily cratered 
than the northern one. This is probably because the north pole pictures have much 



poorer resolution. Erosional debris blankets which mantle terrains surrounding both 
polar zones were probably derived through the continual erosion and transport of 
material from polar deposits to lower latitudes. — L. A. Soderblom and T. J. Kreidler 


AUGUST 1972 

The frost in the north polar region is shown above covering a region about 2700 km 
wide about five or six martian weeks after the vernal equinox (August 1972) and at 
right when nearing its minimal extent approximately two weeks after the summer 
solstice (October 1972). The frost cover had a peculiar polygonal shape that became 
very pronounced during the last stages of recession of the cap. It is more likely to 


have resulted from regional phenomena than from local scarps or ridges. Regional 
textural alignments could have been induced by stable wind patterns. Note the crater 
at 73°N, 198°W in the mosaic at right. It trapped and shielded frost from the Sun, 
leaving a large patch on its floor. — L. A. Soderblom 


REV 11 

REV 231 

(85°S, 355°W; IPL 1312/023810, 7352/184744) 

From November until March the south polar cap was in the late stages of its retreat, shrink- 
ing to a residual cap about 6° in diameter. The conspicuous curvilinear markings seen as 
bright bands in 1969 by Mariner 7, defrosted early in 1971 to become the dark bands shown 
here. These high resolution photographs were taken 110 days apart by Mariner 9. In the 
initial stages of its retreat the windows in the cap continually changed, but by early 1971 
they became fixed and unchanging. The dark features are bare ground, where ice has evapo- 
rated from sun-facing slopes. The permanent cap probably contains substantial water, if it 
is not all water, because a permanent mass of frozen CO2 would collect water even from 
Mars' dry atmosphere. The width shown in each photograph is about 100 km. — L. A. Soder- 

(89°N, 200°W; MTVS 4297-47) 

The martian north polar frost cap approached its minimal extent about one-half martian 
month after summer solstice on October 12, 1972. The cap is about 1000 km across. Its topog- 
raphy and the curved patterns in the interior of the frost cap are interpreted as a series 
of stacked, slightly concaved plates, the upper one of less areal extent, with edges that have 
been smoothed and modified. The individual plates may consist of from 20 to 40 separate 
layers, with an aggregate thickness of perhaps one kilometer. The outline of the residual cap 
and configuration of the interior markings arise from the frostfree Sun-facing slopes along 
which layers outcrop. A dark collar of rougher textured terrain surrounds the smoother polar- 
layered sedimentary complex localized in the central regions of both poles. — L. A. Soderblom 

(82°S, 85°W; MTVS 4247-7) 

Contact between layered terrain and pitted plains (above) is shown in this photograph 
of an oval mesa of laminated terrain nesting on underlying pitted plains. In several 
cases, craters can be seen emerging from beneath the layered deposits along their 
margins. One crater, showing only its rim, protrudes through the blanket of the pitted 
plains in the upper center of the picture. The jagged pits and hollows of a pitted plain 
area are dramatically displayed in the lower part of the view. — L. A. Soderblom 

(71°S, 358° W; MTVS 4234^15) 

Integrated pits (right) are etched into a massive layer blanketing much of the south 
polar region. An underlying rough bedrock surface with partially exhumed craters is 
exposed in the pit floors. Slump blocks on pit walls and dark albedo markings at the 
bases of two or three sunlit walls are particularly unusual. Some pit walls are esti- 
mated to be 500 m high. The plain that is shown here is being eroded by wind action 
into irregularly shaped pits that resemble the markings left on a metallic surface after 
it has been etched with acid. — R. P. Sharp 









aj ,- i'<(' 




"'^■i't ^ 

(75°S, 229° W; MTVS 4213-21) 

Polar layered terrain (left) is one of the most striking martian surface features. From 
the ground the layers may look like many of the mesas in the American Southwest. 
Individual layers are probably from 20 to 50 m thick. Their origin is a mystery. 
Smooth, gracefully sculptured surfaces with gentle slopes are characteristic of this 
terrain. The upper edges, unlike those of slopes in the pitted plains, are rounded. 
Layered terrain is essentially crater free, indicating that it is of relatively young origin 
or recent erosion. The seasonal frost cap is believed to play a part in the formation of 
layered terrain, perhaps trapping dust particles which settle as the ice is formed. — 
L. A. Soderblom 

(83°S, 37°W; IPL 1403/203733) 

This view of the polar cap edge shows outliers of ice resting on a mesa of layered 
terrain area about 80 km wide. Slopes of uniform width and declivity facing outward 
from the center of the residual cap defrost earlier than level areas because of their 
inclination. — L. A. Soderblom 


(80°S, 245°W: MTVS 4167-96) 

These irregularly shaped features, located in the layered deposits of the martian south 
polar region, are probably products of wind erosion. The light colored splotch at far 
left is unusual in that it bears no relation to local topography. Also visible is a crater 
which was at one time buried by the layered deposits, but has now been exhumed by 
the wind. (Area shown is about 90 km wide.) — L. A. Soderblom and T. J. Kreidler 

(86°S, 102°W; IPL 1444/131712) 

Detail of the south polar cap ( right) . This picture was taken late in the mission when 
the cap had reached its limit of retreat. The underlying layered terrain is revealed on 
gentle slopes facing away from the pole. — L. A. Soderblom and T. J. Kreidler 






IS'.!'.' .'.1i.«: ' A. 

(83°S, 53°W; MTVS 4261-19) 

In this high resolution picture, part (70 km) of the residual south polar cap is seen 
resting on a mesa of layered terrain. The patchy appearance of the ice mass occurs 
because it consists of a myriad of disconnected remnants. — L. A. Soderblom and T. J. 



Clouds of Mars 

The Mariner 9 view of another planetary atmosphere 
showed many features that are familiar in the Earth's 
atmosphere. Pressures and temperatures in the lower 
Mars atmosphere correspond to those at heights of 30 to 
40 km above the Earth (about l/200th atmosphere and 
— 70°C). Condensation is a slow process under these 
conditions, but both CO2, the predominant atmospheric 
gas, and water can freeze out and clouds do occasionally 
occur on Mars. The total amount of water in the atmos- 
phere is very small; if condensed to liquid, a thin layer 
ranging from less than 0.01 mm to about 0.04 mm thick 
would form, depending on the season. Although the vapor 
concentration is extremely small in volume, compared 
with the Earth's lower atmosphere, the average relative 
humidity on Mars is actually fairly high. Thus, water-ice 
clouds do form whenever the atmosphere is intensely 
cooled by lifting or by emission of radiation. Extreme 
cooling, to temperatures in the neighborhood of — 127°C, 
causes CO2 clouds to form. 

Cooling to very low temperature takes place in the 
polar regions during winter, and an extensive cloud cover 
forms a "polar hood." North of about 65° latitude, a gen- 
eral haze or fog of CO2 ice crystals forms in the polar air 
close to the very cold ground. This cloud cover disap- 
pears in late winter to reveal a surface covered with CO2 
frost or snow. Between 45° and 55° latitude water-ice 
clouds form at heights ranging up to 20 km. Extensive 
systems of cloud waves form as the atmosphere flows over 
rough underlying terrain. The waves reveal that the wind 

direction is from the west at all heights at this season, 
and they indicate wind speeds ranging from as little as 
10 m/s (about 23 mph) near the surface to more than 
60 m/s at a height of 10 km. There is a transition zone 
between 55° and 65° in which large temperature varia- 
tions occur, and the clouds in this region indicate large 
day-to-day weather changes, similar to those occurring in 
the stormy mid-latitude zones of the Earth. 

Recurrent afternoon brightenings occur in the Thar- 
sis region during summer, and are due to water ice clouds 
which form as heated air rises up the outer slopes of the 
Tharsis Montes. These clouds occur during two seasons 
when the water content of the atmosphere is relatively 
high. Other condensation clouds have been observed over 
Argyre and Hellas, and over the north polar region in late 

Probably the dust storms are the most spectacular 
atmospheric events observed. These range in scale from 
the planetwide storm, which obscured the entire planet 
at Mariner arrival, to "small" storms covering areas of 
the order of 100 000 km- (about the area of Ohio). The 
latter were seen several times by Mariner 9 in the region 
of winter storms along the periphery of the north polar 
cloud hood, and they were also seen in the tropics. Be- 
cause the dusty air is a strong absorber of sunlight these 
storms influence the circulation, and the planetwide storm 
showed a unique circulation regime driven by heating of 
the dust-laden air. — C. B. Leovy and G. A. Briggs 


(19°N, 111°W; MTVS 4098-S2) 
(14°N, 110°W; MTVS 4098-78) 

Some of the last photos received from Mariner 9 showed extensive cloud activity near 
the largest volcanoes on Mars (right). A high resolution picture (above) of Ascraeus 
Mons acquired at the same time showed cells suggesting convection, and the infrared 
spectrometer identified the clouds as water ice. They appeared to be relatively low, and 
were probably caused by air cooling as it moved up the slope of the volcano, but exchange 

of water vapor with the ground or even volcanic venting could also be involved. B. A. 



Arsia Mons* 



Ascraeus Mons 

m'^.:-:'',7r:-vm. immitiK'. 





(15°N, 42°W; IPL 1765/105021) 
(13°N, 42°W; IPL 1676/210508) 

After the global dust storm subsided and the view of Mars from Mariner 9 was gen- 
erally clear, local obscuration by streamers like those shown at left was observed. 
Twenty days later the streamers were gone (above) ; the arrows point to the same 
crater in both pictures. This region is about 650 km wide. Temperatures there were 
high and the streamers originated along terrain irregularities where turbulence could 
enhance the prevailing winds' ability to raise dust. Short-lived, localized dust storms of 
this type are familiar to astronomers as "yellow clouds" and are very different from 
the white clouds produced by condensation. — C. B. Leovy 









• *' 






■',■ • \ ■ 

/ » . 

(48°N, 40°W) 

Clouds appearing on three successive days along the .southern edge of the north polar 
hood reveal a prevailing large-scale wind pattern (repeated craters indicated by cor- 
responding arrows). Intensely cold air covers the northern part of the region shown. 
Some of the wave clouds on the second day of this sequence were aligned in parallel 
bands, southwest to northeast, and individual elements were perpendicular to the band. 
This structure suggests waves produced in shearing flow along the bands and perpen- 
dicular to the small wavelets. This type of structure is familiar in terrestrfal satellite 
photographs of cold fronts and their associated jet streams. On the third day, the 
band system had moved 1000 km to the southeast. This movement is typical for terres- 
trial cold fronts, and martian cold fronts appear to behave similarly. — C. B. Leovy 




— .-^ 



(71°N, 351°W; IPL 7283/213013) 

Mariner 9 sent back some pictures in the northern spring when the polar hood had 
cleared and the atmosphere there was generally very clear. Later photographs (one 
shown at right) showed that the atmosphere was again partially obscured north of about 
45° latitude. Well defined cloud streaks extended south and west from the edge of the 
surface condensate cap. The streakiness may have been produced by strong winds 
blowing off the edge of the subliming polar cap, but this phenomenon is still poorly 
understood. — C. B. Leovy 

(8°S, 95°W; IPL 0083/151448) 

The equatorial region around Tharsis Monies shows a general dust pall in this early 
photo. The peaks of towering volcanoes appear as dark rings at the left, and at right 
the bright outline of a vast canyon complex, later identified as the west end of Valles 
Marineris, can be seen. Observations showed that the canyons are several kilometers 
deep and the brightening here is attributed to the depth of the dust scattering back the 
Sun's light to Mariner 9's cameras. — G. A. Briggs 



,: » : V :' 



'* <*,''i^'<*"' ■ ■■■■ - ■ ■ .- 





(45°N, 85°W) 

(55°N, 73°W; MTVS 4154^93) 

(43°N, 82° W; MTVS 4229-66) 

The Mariner 7 photo at far left shows a white cloud in the Tempe region (arrow) that 
astronomers have noted there for many decades. Mariner 9 returned better views in 
1972 showing parallel corridors of clouds that ranged up to about 30 km in altitude 
(above, left). When viewed vertically later, it was found that a surface ridge about 
400 km long (above, right), oriented roughly north-south, caused the cloud waves. 
Their composition is probably dependent on the wind velocity. Strong winds produce 
oscillations that permit CO2 to condense at high altitudes and water vapor at low, 
warmer elevations. Weak winds permit only the lower level condensation of water 
vapor into ice crystals. — G. A. Briggs 


(63°N, 347°W; MTVS 4210-78) 

This high resolution photograph shows details of the formation of a wave cloud over 
a crater in the north polar region. The wind is blowing from upper right to lower left, 
and a second wave cloud is forming about 40 km downstream. Both wave clouds 
appear to be quite turbulent. The generally diffuse appearance of the scene is caused 
by partial obscuration by a widespread thin haze of condensed CO2 or H2O. The large 
crater stands out prominently because of surface ice or snow (CO2 or H2O) around 
its rim. — G. A. Briggs 



Natural Satellites 

The tiny martian moons Phobos and Deimos ( from 
the Greek for "Fear and Dread") are very difficult to see 
with terrestrial telescopes. They were discovered only in 
1877 by the American astronomer Asaph Hall, and were 
seen as faint points of light orbiting close to their planet. 
Virtually nothing was known about them until Mariner 9 
returned the images shown here. Because their orbital 
characteristics were not known with sufficient precision, 
the first photographs were taken at substantial distances. 
These images were then used for accurate orbital deter- 
minations, which permitted accurate camera aiming for 
closeup photography. 

Phobos, the inner and larger of the martian moon- 
lets, orbits at an average distance of 6100 km (3750 
miles I above the surface of Mars. It proves to be an 
oblong mass about 20 by 25 km in its major dimensions 
(12 by 14 miles). Deimos orbits roughly 20 000 km 
(12 000 miles) above Mars, and is 10 by 16 km (6 by 10 
miles) in size. Because the martian moons are so small, 
their gravity fields are too weak to force them into 
spherical shape. As with our Moon, each keeps the same 
side turned toward the planet. 

Both Phobos and Deimos are heavily cratered by the 
impact of meteoroids. The number of craters appears to 
be close to the saturation limit, which occurs when so 
many exist on a surface that any new craters formed de- 
stroy an equal number of older ones. Rough estimates of 
the ages of satellites can be made by comparing their 
crater densities with those of similar areas on the Earth's 

Moon that have been positively dated by the ages of rocks 
returned by Apollo astronauts. Phobos and Deimos are 
believed to be at least 2 billion years old, and may date 
back to the early history of the solar system about 
4.5 billion years ago. The satellites also serve as a useful 
standard of comparison for the crater densities on Mars. 
This comparison suggests extensive erosion of craters 
1 km in diameter and smaller. 

Both Phobos and Deimos are dark objects; most 
asteroids and meteorites are brighter. The few objects 
that are as dark contain large amounts of carbon or iron. 

Studies of variation in brightness of these satellites 
suggest that they may be covered with a layer of fine 
particles. In the case of the Earth's Moon, such a regolith 
results from the shattering of rocks by repeated mete- 
oroid impact. The gravity fields of Phobos and Deimos 
are so slight that fragments of impact-shattered rock 
would be thrown out into space. But these ejecta would be 
captured by the gravity field of Mars, going into orbit 
about Mars where the weak gravity of each satellite could 
sweep it up again. 

Theoretical studies of the small satellites indicate that 
they need rocklike strength to escape total disintegration 
from meteoroid impact. Since their weak gravity appears 
insufficient to have originally formed them into cohesive 
materials of sufficient strength, it seems likely that they 
were once part of a much larger solid rock, and were 
fragmented by the impact of a large meteoroid. — 
J. B. Pollack 

















^^^^■■^Qj^ ' t T '■ ' ^HjH 











■P % 




^K<* ^1 









(IPL 1579/163600) 

Phobos from afar at a range of 12 500 km (left) reveals only its largest craters. The 
diameter of the prominent crater near the terminator is about 5 km. The long linear 
edge that runs the length of Phobos is probably the result of fragmentation. — J. 

(IPL 83/235451) 

The best view yet seen by man of Phobos is this computer-enhanced picture taken at a 
range of 5540 km ( right ) . The large crater at middle right, near the terminator, 
appears to have at least one small crater on its rim. More than a dozen other small 
craters are visible. The irregular edges of Phobos strongly suggest fragmentation. 
—J. B. Pollack 






'"»♦ I 


(MTVS 4109-9) 

The profusion of craters on Phobos is suggested in this picture, which is also a mini- 
mum-range view (5760 km). Craters as small as 300 m in diameter are visible.-J. 

lii ', I' 





(IPL 1599/200513) 

Deimos, photographed at a range of 5465 km, reveals less detail, although craters of 
all stages of freshness are seen. The old crater in the center is about 2 km across. 
— J. Veverka 


Martian Enigmas 

Many of the martian features seen in Mariner 9 pic- 
tures can be categorized because of their obvious similar- 
ity to features well known and long studied on the Earth 
and the Moon. Others, however, are puzzling. We cannot 
yet be sure whether their characteristics are unique to 
Mars, or whether it is just that the limitations on our 
current understanding of the red planet prevent us from 
confidently interpreting what we see. 

More detailed study will doubtless lead to a better 
understanding. Some features may be clarified if we 

can find natural features on Earth that are analogous, 
and others may be explained if they can be simulated 
or modeled in a laboratory. Thus the present enigmas 
may lead us to a better understanding of the processes 
that operate on the cold, dry surface of Mars with its 
very thin atmosphere and periodic high winds. In the 
meantime, a modest and by no means exhaustive collec- 
tion of these puzzling features is presented here as a 
sampling of the challenges that have been presented by 
Mars. — J. E. Peterson 















• ^,p':^v**v.^V'?'<ri'i'flts.*y!Lfl!uiStf I 

A strange white deposit occurs on the floor of a crater not far from the martian 
equator. Its high reflectivity suggests ground ice but its location makes this highly 
improbable. The deep tapering reentrants and the suggestion of considerable relief 
above the crater floor leads to the inference that it most probably is not a transient 
feature but rather a permanent deposit nov^ in the process of being eroded by the 
wind. The origin of the deposit itself, which is about 18 km wide, remains an enigma. 
—J. F. McCauley 

(74°S, 166°W; MTVS 4269-19) 

An intricate crater (above) in the south polar region displays an arcuate slump mass inside 
the major crater wall. Both the rim and the slump mass are subdued by a mantling; blanket. 
On the crater floor an arcuate scarp appears, which is 35 to 40 km in diameter and compa- 
rable in shape to the big crater. Branching ridges and furrows within may be due to erosion 
of a volcanic construct; dark pattern within may be volcanic ash. — D. B. Potter 

(80°S, 245°W; IPL 326/171411) 

A complex pattern of delicate swirls and irregular dark tones shows in this picture of 
unusual terrain in Mars' south polar cap. The area covered is about 80 by 85 km. Puzzling 
processes, perhaps some interplay of wind deflation of layered terrain, have modified the 
terrain. — L. A. Soderblom 


(2°S, 186°W; MTVS 4209-75) 

Wrinkles on the face of Mars: The smooth plains are sometimes marked by incipient 
collapse or flowage. It may be analogous to the landslips that occur in silty clay beds 
in the St. Lawrence Valley in Quebec. Collapse might come from displacement of sub- 
surface fluids, or from melting of a permafrost layer. Here collapse occurs on the 
flanks of a low ridge that extends from lower right to upper left. — E. C. Morris 

(67°S, 188°W; IPL 1436/130925) 

Double impact craters (below), with rims and floors thinly mantled, boast striking 
breached volcanic cones rising from each floor. Each cone is surrounded by a dark 
apron that could be lava or volcanic ash. — D. B. Potter 

,]ij.ft,. ..-.^ , 



; V ... * 



^4pr*-> "^^^1 







(65°S, 325°W; IPL 7225/14336, 1671/223240) 

Variable features, pictured twenty days apart, offer a challenge to our understanding. 
View at left was acquired on February 4; one at right on February 24. Differences in 
light areas are probably caused in part by clouds. The changes in irregular patches of 


, C:^^«4. 







iiN'' 4. 



extremely dark material may be caused by settling of dust on dark, fresh lava flows. 
Nearly all the craters in these pictures have a central peak or dome, some capped by 
small craters, which is very suggestive of volcanism. — J. E. Peterson 


1' L 


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(54°S. 179°W; IPL 1406/225906) 

The "peach pit" (above, left) : the dark interior mound within this crater is of inde- 
terminate origin. The light materials in and around the crater are probably wind- 
blown sediment. — T. A. Mutch 

(38°S, 120°W; IPL 7081/154812) 

This flat-topped mountain (above, right), about 20 km across, stands more than 1 
km above undulating plains in the southern hemisphere. Steep sculptured slopes indi- 
cate erosional processes are causing escarpment retreat. A complex ring-like structure 
encircles the mountain and resembles a graben — a downdropped trough — along the 
left and top sides but becomes indistinct at right and bottom of the image. A low 
ridge runs from the center of the flat top to the lower right beyond the ring, while a 
fault scarp crosses the mountain and ring structure from left to right. This large 
mountain is of unknown origin, and does not resemble terrestrial volcanic-ring or 
impact features. It somewhat resembles large outliers of chaotic terrain found more 
than 2000 km to the east on Mars. — H. E. Holt 

t35°S, 216° W; MTVS 4248-31) 

Twin volcanic ranges about 20 km long have some unusual features. Tiny craters cap 
the peaks in each range (arrows). The southwest slope of the southwest range has an 
escarpment furrowed by small channels. The other slopes show mass wasting and lobes 
of slide material. The ranges lie within a very large crater not shown here. — J. W. 


(43°S, 356°W; MTVS 4149-15) 

Three unique features lie in the low resolution area shown below. They are large 
crater-like depressions of unknown origin. Being closed forms, they cannot have been 
caused by fluvial erosion, and their depth and steepness of sides rules out wind erosion 
as the sole cause. Some mechanism of collapse controlled by fracture systems is prob- 
ably responsible, but these features are still very puzzling. — J. E. Peterson 

(49°S, 358°W; IPL 7205/184628) 

Curving within a 100-km crater is a 60-km long depression, the end of which is shown 
here (right, top) . Its walls are very steep, and there appears to be a flat-lying resistant 
layer at its rim. It is about 7 km wide at the arrows. Clouds partly obscure the pic- 
ture. — J. E. Peterson 

(45°S, 356°W; IPL 1943/201557) 

A central plateau in this unique 85-km diameter feature is connected to the surround- 
ing plains (right, center). The steep-walled depressions are somewhat sinuous but 
follow a roughly circular outline. This picture was acquired near the end of the 
global dust storm and is somewhat obscured. — J. E. Peterson 

(50°S, 357°W; IPL 7455/235030) 

A deep linear depression is terminated (right, bottom). It is about 5 km wide at its 
narrowest point. The sides are very steep, with debris-avalanche chutes on the walls, 
but the bottom seems fairly smooth and rounded in cross section. Again, clouds 
apparently obscure this picture somewhat. This gash extends nearly across a large 
crater. — J. E. Peterson 




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(20°N, 235°W; IPL 1765/212715) 

(18°N, 235°W; IPL 7256/131906, 7256/133526) 

This strange feature (above), Hephaestius Fossae, is located in the Elysium Planitia. 
It is a system of branching troughs approximately 525 km long and 75 km wide. The 
elongate shadow of Mars' moon Phobos can be seen in the low resolution photo (the 
round black spot is a camera artifact). In the high resolution mosaic, the general pat- 
tern of the troughs indicates fracturing as a more likely cause than fluid flow. Indi- 
vidual troughs are up to 2 km wide, and considerable erosion by wind may have 
broadened them. — R. S. Saunders 

(9°N, 293°W; MTVS 4266-35) 

An enigmatic collapsed depression (left) occurs in the region of Syrtis Major Planitia. 
The structure here has the crisp arcuate scarps that characterize the volcanic calderas 
of the Tharsis Montes. But it may also be a karst-like feature formed by removal of 
deeply buried ice or other subsurface materials. (A karst is a region of sinks and 
ridges overlying limestone.) — T. A. Mutch 



Mars, Earth, and Moon 

It is impossible to look through the thousands of 
images of Mars returned by Mariner 9 without discover- 
ing features reminiscent of those on our native planet. 
When a match-up is made by geologists having profes- 
sional familiarity with the Earth's features, as on the 
following pages, the similarities can be striking. The 
Moon, liberally photographed during the first years of 
space exploration, also has details that appear similar to 
those that Mariner 9 revealed on Mars. Sometimes labo- 
ratory simulations can help bridge the gap between the 
apparent and the known, as when wind-tunnel experi- 
ments on model craters seemingly duplicate erosion pat- 
terns seen on the surface of Mars. 

For two reasons these analogs may be less surprising 
or significant than they seem. In a sense it is small wonder 
that Mars, Earth, and Moon do, in fact, look somewhat 
alike, particularly if you examine the Earth through 
geologists' eyes. Windblown features are easily identifi- 
able in, for example, Texas. Idaho, and New Mexico. 
Evidence of volcanism can be seen in Arizona, faulting 
in California, stream erosion throughout the United 
States, stratified deposits in Utah, and glacial features in 
Alaska. The Hawaiian Island complex is comparable in 
some respects to the volcanic region of Mars near Olym- 
pus Mons. 

We should be cautious and not make the mistake of 
assuming that resemblances — limited as thev are to the 
physical appearance of surface features — are proof of 
true similarities. It can be a profound mistake to assume 

that similar-looking features actually originated and 
evolved in a like manner. Without a doubt, future ex- 
ploration of Mars will show that some of the dynamic 
processes that shaped the surface of Mars were the same 
as those that caused terrestrial features. Geologists are 
now conducting research programs in the southwestern 
United States, Peru, and Antarctica to collect data that 
may cast light on the question of whether Mars and 
Earth evolved similarly. Theoretical calculations and 
laboratory experimentation will provide the quantitative 
information needed to understand the physics of these 

The exciting thing about comparative planetology is 
that it will permit us to unfold the lost part of the 
Earth's history, now largely obliterated by erosion, moun- 
tain building, and other processes. A full understanding 
of the past is a reliable way to accurate prediction of the 
future. This work can help predict the nature and course 
of future atmospheric evolution, answering the disturbing 
question of whether the Earth's environment is destined 
to grow similar to the environment of Venus or Mars. 
Questions like these can only be approached by compre- 
hending the secrets of the planets in our solar system. 
Comparative planetology is the starting point for an 
understanding of the physical future of planet Earth. 

In the meantime, the analogs on the following pages 
suggest that the old saying may have to be modified to 
■'It's a small solar system." — S. E. Dwornik 


(14°N, 142°W; MTVS 4174-75) 

Wind-produced streamlining of a part of the complexly structured aureole around Olympus 
Mens is seen above. These elongate ridges are 10 to 15 km long and 3 to 5 km wide. They 
are parallel to numerous smaller grooves and roughly elliptical pits that are also the probable 
result of wind erosion. The crests of many of these ridges occur sharp and keel-like in appear- 
ance; their ends are sharply tapered. These ridges occur in terrestrial deserts such as Iqa 
Valley in Peru (right) where they are several kilometers in length and hundreds of meters 
high. The Iqa Valley ridges have been cut by strong sea winds that funnel almost daily into 
this virtually rainless valley. The layered rocks here are relatively soft. Tertiary sediments 
uplifted from the sea by faulting since the onset of aridity in this region. — J. F. McCauley 





(81°S, 64°W; IPL 1417/160341 1 

Another probable result of wind erosion (below ) is seen in this unusual and complex 
array of linear, interconnected reticulate ridges in the south polar region of Mars. 
(The picture is about 45 km wide.) A superficial resemblance to ancient ruins led to 
their informal appellation as "Inca City" during the Mariner 9 mission. A more mun- 
dane explanation is that this feature almost surely represents yet another variant of 
the landforms produced by wind on Mars. The origin of the reticulate pattern itself 
is unknown; igneous or clastic dikes or indurated fracture zones are all possibilities. 
As can be seen in the photo from the almost rainless coastal desert of Peru (right), 
similar patterns can be produced by selective wind scouring. (The image is about 21/^ 
km across. I The more resistant dikes or fractures abrade less rapidly than the softer 
surrounding material and thus stand above the surrounding plains like the walls of a 
ruined city. — J. F. McCauley 





(18°N, 133°W; IPL 1406/164237) 

Calderas on Earth are created by the collapse of the surface as lava is erupted or when 
it drains away at depth. Here repeated collapse events produced complexes of older 
large calderas surrounding a smaller younger one. Shown are Kilauea in Hawaii 
(above) and Olympus Mons (left) on Mars.— K. A. Howard 







(1°N, 157°W; MTVS 4254^55) 

U-shaped depressions are often produced by the wind in the lee of rocks or other 
topographic obstacles. These occur where an active, mobile sheet of loose-moving sand 
is present on the surface. Deposition of the sand tends to occur on the upwind side and 
the flanks of the obstacle and an erosional blowout or depression occurs on the down- 
wind side. These features may be controlled in great part by the presence of partially 
buried crater rims just now poking above the sand blanket. Picture at left is a high 
resolution image of the Amazonis Planitia on Mars; blowouts shown are up to 3 km 
long. The picture above is a low-altitude aerial photo taken in the Coachella Valley, 
California, where the blowouts are tens of meters long and occur in the lees of 
abandoned shacks. — J. F. McCauley 


(6°S, 84°W; IPL 1356/114237) 

Stubby, relatively deep gullies without well developed tributaries are seen in the photo at 
right of an alluvial wash on the shore of Lake Mead, Arizona. They are developed in 
loosely consolidated material that fails by slumping and soil flowage due to changes in 
the lake level and degree of saturation of the soil. A similar stubby, poorly developed 
dendritic pattern (above) is seen in many tributaries of the Valles Marineris on Mars, 
suggesting that they may have formed by some type of sapping or soil ffowage process 
rather than by water collected runoff from rainfall during an earlier pluvial episode on 
Mars. — J. F. McCauley 



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(2°N, 314°W; MTVS 4178-60 

Central peaks in craters or basins, though widely assumed to have been formed by 
impact or volcanic processes, can also arise from wind deposition. While the origin of 
the peak in the Mars crater (left) is as yet unproven, in the basin shown above at 
Bruneau, Idaho, the center is dominated by a large sand-dune complex maintained by 
wind blowing in two main directions. — J. D. Murphy, J. S. King, and R. Greeley 


(56°N, 16°W; IPL 1643/194728) 

Clouds on Mars can resemble those on Earth. Flowing past a frost-rimmed crater 90 km 
in diameter, northern winter winds form clouds of a characteristic lee-wave pattern 
on Mars (below). At right, a similar lee-wave pattern was seen by Nimbus 1 down- 
stream of the Andes over Argentina. — C. B. Leovy 

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(60°N,270°W; IPL 1431/193738) 
(60°N, 178°W; MTVS 4248-98) 

Long cloud lines on Mars (left, top) are formed by convection as cold polar at- 
mosphere rushes southward over warmer ground. The convection creates long spiral- 
ing plumes downwind, with clouds forming on the rising part of the spiral. At left 
bottom, similar cloud lines begin to break up into large convective clusters in another 
part of the martian north polar region. Above, an Apollo photo shows cloud lines on 
Earth, where relatively cool air from the Atlantic flows northward over the warm 
ground of South Carolina. — C. B. Leovy 




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(25°N, 213° W; MTVS 4298-40) 

Radial structures about this summit caldera in the Elysium Planitia of Mars (left) are 
interpreted as fractures that have been modified by lava flows. Compare them with 
the similar fracture from the rim of the Mauna Loa caldera, pictured above. (A small 
segment of the caldera rim is in the upper part of the picture. ) — R. Greeley 


(38°N, 196° W; MTVS 4244-76) 

Martian inselbergs near Phlegra Monies (left) and terrestrial inselbergs in New 
Mexico (below). In dry climates these eroded remnants of mountains are sometimes 
surrounded by bajadas (debris sheets). A good example is seen in the upper right of 
the Mars photo. Most terrestrial mountains are eroded gradually and smoothly both 
by wind and rain; debris is washed evenly onto the surrounding area. However, in 
deserts infrequent but voluminous cloudbursts are responsible for the transport of 
great quantities of rock materials that accumulate as a depositional apron around the 
inselberg in Animas Valley, New Mexico. Bajada-like features are also seen on Mars; 
their origin is uncertain since there is no present fluvial activity. Perhaps these debris 
aprons are related to the desiccation that is evident from dry stream channels. The 
bajadas also may have formed simply by gravity as the debris slid down the slope to 
flat areas. The terrestrial inselbergs shown here are approximately 1 km in diameter: 
the remnant in the upper right of the Mars photo is about 10 km in diameter including 
the bajada. — W. E. Elston 

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(9°N, 191°W; IPL 1947/173205) 

A dark plume (left I extends more than 140 km downwind of this large crater in the 
Elysium Planitia. A laboratory simulation above, with the wind flowing from top to 
bottom, suggests that the dark martian plume may have been caused by wind erosion 
removing loose particulate material. Alternatively, the dark plume may be deposits of 
material originating from within the crater. — R. Greeley 


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Wind-tunnel simulation examines effects of low-velocity wind flowing (left to right) 
past a raised-rim crater. Note the development of the blowout on the downwind flank 
of the crater. Such tests must consider wind velocity, crater geometry, threshold char- 
acteristics of surface material, scaling effects of size of crater, and effects of martian 
environment. — R. Greeley 


(16°N, 182°W) 

Very large, irregularly shaped craters exist on Mars (above) and on the Moon (right). 
The martian crater, Orcus Patera, is more than 400 km long; the lunar crater, 
Schiller, is about 180 km long. Craters of this shape and size are uncommon and their 
origins uncertain. Coalescing subcircular segments marked A suggest they may have 

been formed by overlapping impacts or as volcanic features, but linear scarps and 
troughs marked B indicate a tectonic influence. Floors of both craters are flat and 
smooth. The martian crater is probably floored with wind-blown dust; the floor of 
Schiller may be covered by impact ejecta and volcanic flows. — D. H. Scott 

(31°N, 220°W; IPL 1443/140643) 

Highly elliptical craters on Mars and the Moon. Note that both the 12-km-long Messier 
A lunar crater (right, bottom) and the 15-km-long unnamed martian crater (below) 
have raised rims, linear structure on their floors, and ridge-like topography outside the 
long axis of the ellipse. While certain of these features have been interpreted as evi- 
dence for volcanic origin, laboratory studies have shown that the observed features 
can be reproduced in detail by low-angle meteorite impact. — N. W. Hinners 


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(13°N, 107°W; MTVS 4184-75) 

Interrupted rilles appear both on Mars (left) and on the Moon. In each case low-ele- 
vation terrain adjoins high-elevation terrain, and both are transected in varying degree 
by rilles. Some of the martian rilles, such as those near the sinuous channel at upper 
left, may have been filled by deposition or sediment. Parts of the lunar rilles seem 
to have been somewhat filled by later lavas. Scale: the middle martian rille has an 
average width of 700 m; the largest lunar rille shown is about 2 km wide. — N. W. 





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i (23°S,204°W; IPL 1642/1883351) 

Heavily cratered terrain on Mars (above) bears striking resemblance to some areas on 
the Moon (right). One notable difference is that Mars does not appear to have as 
many smaller, bowl-shaped craters, which leads to the inference that, on Mars, they 
may have been eroded and filled. — N. W. Hinners 

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(31°N, 192°W; IPL 1449/223730) 

Similar valleys on different worlds: a martian valley near Phlegra Monies (below) and 
Alpine Valley on the Moon (right). Both cut plateaus are studded with rugged peaks 
and the valley floors are filled by smooth plains materials. The Alpine Valley, 130 km 
long in the photo, belongs to a radial fault system in the Imbrium basin rim; the 
plains materials are post-basin mare basalts. The martian valley (photo width is 55 
km) could also be a fault graben, but no relation to a basin has been discovered, and 
its origins are uncertain. — D. E. WilheLms 


■^N >■ 

of Photographic Prints 

Throughout this publication Mars imagery is identi- Information and price lists for general interest re- 

fied by MTVS or IPL numbers except where mosaics are quests for any photo in this publication may be obtained 

presented. These numbers represent the best processed from 

image available. d du » u;„ i 

'^ ri r 1 i_ c f Bara Photographic, Inc. 

NSSDC has Mars photos on file for the benefit of p ^jr- r. 4^05 

scientists engaged in the study of Mars. Inquiries (for Bladensbur", MD 20710 

MTVS or IPL numbers only) should be directed to 

National Space Science Data Center Orders should include the publication number (NASA 

Goddard Space Flight Center SP-329) and the page number (indicate "top" or "bot- 

Code 601 tom" where necessary). 
Greenbelt. MD 20771 




Polar Stereographic Projection 




4000 km 




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Mercator Projection 

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